This game is probably rated right, but definitely underated as Wii game. One piece fans will definitely enjoy this.
There are 7 characters that you can swap at any time with a fair number of combos that can be unlocked for each. These involve swinging the Wii remote and pressing various buttons. The characters each have different moves suited to different enemies and bosses.
The normal/common enemies are usually at the correct level when fighting them at first, but obviously after leveling up you can destroy them later on. The bosses are about the right level of difficulty, but do need some amount of strategy to be defeated.
Other than the standard adventure stuff; there are other mini-games that are fun if you get bored or just want to try something different. There is fishing at various spots and by improving your rod you get better and bigger fish. The bug catching is very similar, with better nets being able to contain stronger(?) bugs. Items can also be collected and combined with fish or bugs to create items or dishes. The dishes increase the stats of characters and items advance you further. All of this is collected in a scrap book of sorts.
Overall the game is quite good and in my opinion is one of best Wii games. It has a feeling of fan service, but backs it up with some pretty good gameplay. A must have for anime fans and hardcore/ casual gamers, as most Wii games are family orientated.