It's weak. It didn't hold my interest. There are a couple of decent bits with Luffy, but basically it's a summarised retelling of TV series episodes that are probably more effective in their original form. It's doing so from the point of view of Sabo (hence the title), but unfortunately this was a big surprise character for long-term fans and so he spends most of the film hanging around watching from the shadows.
I have a crotchety relationship with One Piece. It's wildly successful and one of anime's mega-franchises, but I disapprove of it. Everyone in it is a pirate. They've wanted to be a pirate all their lives. Being a pirate is heroic, if you believe the implied message. The franchise has always skated over the moral objections to this by never letting good pirates commit any piracy, but I call that a cheat. In fairness such concerns aren't generally a problem when you're watching an actual episode, which will generally be turbo-charged family-friendly heroism... but at times I found myself feeling a bit uneasy with this TV special.
You see, it has two parallel stories. One is set in the present day, with Luffy and his friends fighting baddies in the slightly surreal country of Dressrosa. The other, though, is set long ago when Luffy, Ace and Sabo were children playing together and vowing to be pirates one day. The latter sat slightly uncomfortably with me.
(The other thing I dislike about One Piece is Nami, but fortunately she's hardly in this and so I didn't need to hate her.)
This special's retelling the first third of the Dressrosa Arc (eps.629-746), which is the anime's 27th story arc. That's roughly a third of 118 episodes, i.e. 2 minutes 45 seconds per episode. Don't expect detail. Dressrosa is quite a nifty place, though, with living toys walking among the people as if we're watching a Disney cartoon. The toys are cute. They're meant to be funny. However their existence becomes sinister when we learn that most of them used to be human, until they drew the attention of a certain Devil Fruit user.
Dressrosa is actually ruled by a small group of Devil Fruit users. Their favoured amusements include gladiatorial contests in the Corrida Colosseum. Guess what Luffy ends up doing.
There's lots of fighting. There's relatively little involvement for most of the show's regular cast, who are so numerous by this point that the story has to split them up into three groups just to keep the storytelling manageable. There are over a dozen of them. None of them manages to be memorable, except visually. (The hulk with pigtails is a striking character design, even if I don't remember anything he did, while all the women look like Jessica Rabbit.) I didn't really care about any of it, although I do admire the show's favourite trick of adding weight to fight scenes by making Luffy annoyed about something bad his enemy has done. That works rather well with the bull.
The childhood flashbacks have a stronger focus on the characters and in that sense are more successful, but their storyline doesn't matter in the slightest.
1. If you receive a mortal blow in combat while shielding your childhood friend with your body, then the enemy who just punched his fist through you will disappear with no indication of where he might have gone. You'll then have a long, long, loooong death scene with no interruptions.
2. One of the Coliseum participants is a girl with huge boobs and a bikini that doesn't join up at the front, instead being held in place by wishful thinking. I wouldn't fight in that. Nonetheless she survives Round 1, being the last warrior standing in a free-for-all of hundreds of savage killers with superpowers. She then becomes a damsel in distress in Round 2, to be protected by heroic men.
Maybe I'd have liked this more if I'd seen the original episodes? It has a couple of good moments, when Luffy's angry with a baddie. I liked the living toys. However as a whole it struck me as hollow, making insufficient effort to make its story work in its own right. I didn't care about its characters, none of whom earned any emotional investment. (Luffy's cool, but that's it.) The storyline had me reaching for the fast-forward button. I could imagine One Piece fans enjoying this TV special, but probably because they'd already seen everything in it and so were basically rewatching something that was already inside them. Non-fans, though, should run a mile. It's terrible. It's not meant for you. It's been demonstrated repeatedly that it's possible to make fun, accessible One Piece movies, but this TV special is the storytelling equivalent of an ingrowing toenail.