When I'm not doing my best Amanda Bynes impersonation, I'd like to think that one of my more innocent vices is buying into hype, feeding it, and getting caught up in the afterglow of something fresh in my mind. I regrettably did this after Strong World, and while I didn't dislike it, I feel like I gave it a more positive review than I should have–I may have liked it more than I usually would have simply because I felt that I had to. I'm not going to do that with Z, especially since I went in with pretty moderate expectations. So this is just a disclaimer that I still have the movie fresh in my head, but I don't actually feel as if that's pushing me in any one direction. Light spoilers below.
! I think what I enjoyed most about the movie was that it seemed to have a good idea what it was, and thus had no real problems in terms of pacing. It's a big fat One Piece movie, but handles obligatory explanations pretty well and doesn't bog itself down with them. At this point, all the moms and dads in the theater know what's what, so they don't need constant reminders. I feel like a lot movies out of major anime series need to do this, so take it as One Piece taking advantage of its own popularity.
! Character banter is pretty good. Things like Chopper using a milk bottle to mimic Z's sea-stone arm, Zoro calling Sanji disgusting for celebrating a chance to watch her go through puberty, and Robin/Nami/Usopp/Chopper running a bellydancing-shoeshining ring to collect info on Z–remind me of the charm OP has, where you have chapters with absolutely no action or major development, but still walk away really satisfied. Not to say that there isn't action--the fights are incredibly well animated and go on for some very impressive uninterrupted sequences. Every crewmate gets a chance to shine, but as has been said, it's mostly the monster trio and Franky at the end that do most of the work. Nami and Robin fanservice was a little embarassing and over the top (I don't watch the anime, so I'm not sure how unusual that is), if Brennen was in the theater, we could have had a serious Pee Wee Herman incident on our hands.
! I enjoyed the marine lore. Seeing the admirals when younger, hearing about Z's backstory and whatnot is a nice change of pace from the Roger/Shiki/WB stuff we've been given. One thing I appreciated is that there is a lot done to make Z more than just a bitter old man that wants to stomp things--his backstory, comments throughout fights, and the very last scene validate him and his cause as something to take note of. In fact, I'm going to talk about the last scene in a bit of detail, so don't read any further if you don't want to be completely spoiled--I think it's been mentioned, but this is just my take on it.
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! Throughout the movie, Z proudly boasts his name and looks down on Luffy for having a dream to be something as big as the Pirate King. He seems very sentimental about dreams and aspirations, but until the last scene you just write this off as a typical character trait that Oda has a fetish for. In the last scene, after his death, there is a small flashback to Zephyr as a kid, dressed up as a super-hero, play-battling his friends. He throws up his hands to form a Z and calls himself that. You get the impression that this is where he first came up with the moniker, and the aspiration to be a true hero and fight for justice as a marine. Once you connect that with the way that he was betrayed and fought against both pirates and marines for what he truly believed was justice, under the name of Z, you can piece together a character backstory that fits in with all the great ones Oda has come up with.
! It may sound like I am gushing over this movie. In a way I am, as I really enjoyed it and in part because I was surprised by it. It's not without its faults, however. As I said, the fanservice is silly, some characters are under-utilized, and this is my own personal quirk–but I'm not a fan of contrived setups to get the crew in different outfits. I don't see the point of a one-eyed person who doesn't wear glasses to suddenly put them on. It's anime, though, I try not to take it too seriously. Also, my biggest complaint--Ain and Bins are pretty fucking lame. Ain's power is creatively used in fights and used as a plot point, but outside of that the duo annoyed me more than anything.
! So yeah, I liked it. Probably my favorite OP movie. It delivered what I wanted--a big fat OP movie with a good balance of aesthetic fluff and added to the story/lore of the series. That's what I wanted, so if that's what you're asking for I think you'll be happy. I only had two beers so I think I didn't embellish too much.