Though I give the series a 10 hands down, this isn't as good due to the time constraints of a movie. No story arc in One Piece is told in this little amount of time, so it feels very rushed compared to it's normal baseline for story arcs. Really what drives this movie is the "cool" factor. You get to see the amazing Straw Hat Crew dress up in new outfits and kick a little but. It's worth watching, but it's not up to normal One Piece standards. That last comment aside, it's still the best movie, and it is still pretty good.
If you haven't seen the One Piece series, then I wouldn't watch this yet. Like all movies based on anime series they skip the character development. It works in this movie because the characters have such strong and unique personalities that you get to know them very quickly, but this is a side story and the anime series is a flat out masterpiece.
That aside, it's worth watching, and it has high repeat value.
