I haven't played One Piece Unlimited World Red but did play the other games in the Unlimited series and they were a lot of fun, specially the Unlimited Cruise games. They're basically beat em ups with light RPG stuff, like leveling up and item collecting and crafting. Like in most of these games, things might get a bit repetitive at times, but there are a lot of playable characters with their own combos and abilities, plus a lot of service for One Piece fans like me. The animations, character models and voices (in Japanese, at least) are fantastic. The music is nice as well. Ganbarion is the developer and they are cool in my book for making the Jump Ultimate Stars games on DS and Pandora's Tower for Wii. They have a knack for making games that "feel right" to play.
Again, I haven't played this particular game but it seems to be very similar to the previous games in the series, only bigger and very possibly better. For that price, I say yeah, go for it, specially if you like One Piece as much as you should.