This is going to be very long (like the series in question).
I've come to generally "trust" One Piece. There's been many times my enjoyment of the series has been reduced, but it has always managed to pull me back with stuff that makes me interested and impressed all over again. Tomino (Gundam creator), who has talked about OP a few times, once said:
"But, you know, among the popular manga there is manga with very beautiful art and manga with bad art, but interesting nonetheless. And I don’t trust manga with very beautiful art unless it is One Piece.”
I began reading One Piece in high school (2017, I believe) through my school's library, which had an extensive manga catalogue, including what I remember as being most of One Piece. Before that I was a Dragon Ball fanboy (if you stalk my account enough you'll see I used to argue my brains off for DB on r/whowouldwin) who had also tried (though often not finished) other stuff including Hunter x Hunter, One Punch Man (anime and webcomic), Gurren Lagann, and Akira (movie and manga). At the time the manga was in Whole Cake Island. I didn't know much about the series, but I knew it was big and I had seen it referred to as being among the "Holy Shonen Trinity" (now commonly known as the Big Three). When I started it, I found it charming and cool. The concept was appealing, the main characters were likeable, the stories were entertaining, and the SBS section was fun. The art, while very different from most stuff I had seen prior, was also very appealing to me. I remember reading Baratie in band class and loving the action (Luffy breaking Don Krieg's armor was a striking moment to me); it was both cartoony and over the top but hard hitting, kinetic and brutal. When I got to the classic "Help me" moment in Arlong Park I was enamored. I kept reading, very much enjoying most of what I read up until Skypiea, where one or a few volumes were missing in the library. I wasn't as hooked and took a break for a while, and when I came back I made the dumb decision to just read the next volume available, finished Skypiea, and then picked up toward the end of Foxy when I made the switch to digital, missing crucial parts of Skypiea and most of Long Ring. I won't go into an extreme amount of detail of the process after that since I've already talked a lot, but I will say that I enjoyed most of everything that came after, and caught up during Wano. By then it was pretty much cemented as my favorite series. The fact that it was long usually wasn't really a problem to me because I wasn't watching Toei do an impression of a snail and I wasn't clamoring for the series to end.
Over the past several years, One Piece has become highly scrutinized online and has lost a portion of the good will it once carried in the public eye. A lot of this criticism is understandable and expected. There are many problems with One Piece, both lasting and more modern. I myself had become a bit unenthused when I caught up in early Wano, and since then there's been tons of polarizing stuff, including something that Oda can be seen to have anticipated to be controversial, Gear 5. But I also believe that a lot of the common criticism levied towards this series has become a series of lazy, misleading cliches and misrepresentations (largely brought on by the fact that a lot of people either haven't read the series or stopped a while ago). There's an increasingly common perception that One Piece is a low-effort, mass appeal product of unremarkable quality meant to go on forever (a.k.a slop). On the contrary, I truly believe that on balance, One Piece tends to be a high quality series, especially when you compare it to its peers.
First, what are the series' main strengths? I'd say: a knockout main character, a very wide variety of great ancillary characters, strong thematic writing, remarkable emotional resonance, relatively good internal consistency and continuity (especially considering its length and density), great worldbuilding, attention to detail, solid creativity, "vibes," and a story and world that gives a lot to talk and think about. I think that in general, even though these things will waver throughout the story, Oda's skill in these areas remain to this day. I also think that compared to most of the other stuff that would be most likely to be compared to it, One Piece is a monster in these departments. JJK is a high quality series that is absolutely huge, but it would be hard to defend the opinion that Yuji is a a better main character than Luffy for most people who enjoy both series, valid as the opinion may be. Naruto is extremely solid, but I think OP reaches higher peaks and will not have an ending arc as problematic. I got bored with Bleach in a way I didn't with One Piece. I just think it's genuinely distinguishable in quality from a lot of stuff.
Now there's so much stuff I could rant about when it comes to discussion of this series, but I'll get into some prominent topics of contention where I think there are misrepresentations. First, I actually think that One Piece's "politics," or other similar "serious topics," while notable, are not as central to its quality as a lot of the fans will say. There's tons of stuff with similar audiences that addresses that type of stuff, like Fullmetal Alchemist or Avatar the Last Airbender. I'd say in terms of themes, One Piece is actually way more noteworthy "philosophically" or "poetically" than politically. Take the arc of Skypiea, for example (which I went back and fully read, along with Foxy). There clearly are themes that could be called "political," such as claim to land, progress vs. tradition, and the struggle against the erasure of peoples and history. But there should also be talk about the themes of romance (in the literary sense) and dreams, faith, and the human desire for vindication. Don't get me wrong, One Piece has politics. But I think the incorrect framing of One Piece as a battle shonen Communist Manifesto rather than something more like a battle shonen William Blake work actually leads not only to misplaced discussion, but improper criticism! You'll hear stuff like "it's not that deep, One Piece is just dictators bad, slavery bad, racism bad, starving bad" as if the series wasn't saying tons of stuff before the revolutionary themes were even that prominent. As if the most interesting part of Fishman Island isn't that it tells you that racism is bad, but how it explores how discrimination affects a people's spirit, and the paths of healing for that. I mean why, with a series with so much analysis and discussion of parallels to real life, am I the only person I can find who has noticed the possible connection between Shandora (golden city that ascends into the heavens) and the New Jersualem from Revelation (golden city that descends from the heavens)? Especially in an arc with so much talk of "God?" People have pointed out the Israel/Palestine connection in the arc; why not this other possible theme connected to Abrahamic faith and history?
One Piece nowadays often gets flack for its female characters. For example, a viral tweet that has inspired much discourse and many derivatives posited what it would be like if Oda had wrote Maki from JJK. I won't deny that One Piece is primarily a boy's club, and that there's notable fanservice and a repetitive design template for pretty female characters. However, let's look at the actual common claim: that Oda is bad at writing women. Is this really an accurate claim, at least when compared to other shonen, and even JJK itself? When I think of One Piece women, though there's not as many super important or strong ones as the boys, I think of characters that are generally good, if not always in execution, then at least in their construction. Nami, Robin, Vivi, Pudding, Bonney, Big Mom, Carrot, and Hancock are all characters I have positive opinions on. Robin's pre-TS arc would probably be the best character arc in many series, period. People used to directly compare One Piece's relatively good female characters with series like Naruto, though there's long been criticism levied towards designs and sexualization. And with all due respect to Gege, he would have an aneurysm trying to write a character like Kureha. I think it's funny that Maki is used as the example because if you take any other JJK female character, they'd have less prominence, less notable writing, little in terms of a character arc, and likely have a far smaller fanbase. I mean, Gege often got these same "bad women" allegations! As did Horikoshi! I'd sooner accept Oda as a disgusting misogynist than as someone who couldn't write women, because I think that's blatantly untrue. Toriyama has even more direct proof of misogyny ("Lady Red" is pretty insane, content warning), has even more egregious fanservice in early DB, and his series is even more of a boy's club, but I'd he say he can write women too.
As I mentioned before, One Piece will have points where I start to enjoy it less. However, these instances are then punctuated with peaks I just can't get anywhere else. Thriller Bark, which isn't a bad arc in my opinion but is understandably less popular, ends with an absolutely insane sequence when Kuma arrives and is followed by the freaking Sabaody to Marineford stretch. Wano had tons of polarizing stuff, but it also has what might be the best fight in the series, an instant classic Roger moment that redefined what we thought of the One Piece, tons of wow factor, lore stuff, great character stuff for Sanji, and as a whole was a big part of One Piece's new wave of mainstream popularity in the West. Most recently, I personally was not as motivated to read recent Elbaf, though on paper I liked the turns of events. But the recent Sanji stuff is extremely compelling and rewarding, and now I'm really excited for the next chapter. The idea that Oda is just washed and can't make something worth reading anymore is just not true to me, and if you've seen that screenshot of someone ranting on a forum around the time of Enies Lobby complaining that the series had lowered in quality, you've seen that this type of discussion is not new. On that note, when I was reading Naruto a while back, I tended to look up old discussions for chapters after finishing them, as I often do for manga. I already knew that there was tons of criticism, but man, the discussion was brutal. It was very often filled with a significant amount of negativity, even concerning stuff that now is lauded, like Itachi or Obito. This fall from grace posture might just be a feature of the long running shonen space in the information age.
Now, One Piece certainly isn't without its flaws. Some of the main ones I'd point to are:
- Repetitiveness. I don't mean the cliche people bring up about Luffy saving a kingdom/island and princess from a dictator/oppression; One Piece actually deviates from this a lot (many of its most popular or critically acclaimed arcs don't follow this, including Baratie, Water 7, Enies Lobby, Jaya, Sabaody, Amazon Lily, Impel Down, Marineford, Whole Cake Island, Zou, Egghead) and even when it doesn't, the world and wider themes justify liberation from oppression being a common theme. I'm referring to stuff like the narrative forcing the Straw Hats to separate so that they can do a bunch of tasks and maybe have individual fights. Stuff like the forced "Ticking time bomb" element added to a lot of arcs added for tension. Stuff like Luffy conveniently befriending someone from almost everywhere he goes to give him a reason to risk everything for them while the narrative insists he's not a hero. Stuff like gags being repeated without enough variation.
- Fakeout deaths. Self explanatory, though Oda has been killing more people in recent years.
- Decreased focus on the Straw Hats, and a general feeling of flanderization there in general for a bunch of them.
- Bad paneling after the timeskip (though recently it's been more serviceable)
- Art is just not as good anymore. I think the peak of Oda's pen was probably early to mid post timeskip; things were just really sharp and pretty. Understandable; he's getting older. But it's quite noticeable.
- The fight choreo and flow is inconsistent, especially over time. There's some genuinely really good stuff and some really mediocre/bad stuff. Wano actually has some of Oda's best (read 1010 or 1037 again) and then after Wano it's just not as good. This seriously hurts an arc with a good deal of impressive writing like Egghead, because other than that it's largely a mad scramble of fights.
- Related to the fight complaint: the chapters nowadays almost feel like they're intentionally written to be adapted into the anime. As if Oda realized that Toei was putting more effort into the anime, and also that they're only going to adapt one chapter, so the best thing to do was to write them like a storyboard for an episode, bloated with text, and with deemphasized action art. Like he thought "who cares about making my chapter a good comic when the anime will make the actual fights and have cool directing?"
All in all though, I think One Piece is really good for what it is, and I trust that the ending will be good too.
TL;DR: I trust One Piece. I got into the series years ago after already experiencing other series and it captured me like very little before it. I think the increased scrutiny its gotten, while largely understandable, is also kind of inaccurate in ways. Despite its flaws, One Piece is peak battle shonen and will likely stick the landing.
