Let me get this straight – I love One Piece, but 4Kids has done terrible things with the license. Still, suffering through the horrid dubbing and edits is the only way we're going to get the games over here (the most promising of which looks like One Piece: Unlimited Adventure for the Nintendo Wii).
One Piece: Grand Adventure is the sequel to last year's Grand Battle, or rather an extension of such. The gameplay is fundamentally the same and plays similar the cult Capcom fighter Powerstone. You and the enemy run around in a small environment pounding on each other, avoiding environmental obstacles, and collecting items. It's more dynamic than a traditional fighting game, but not quite as deep.
The revised Adventure Mode allows you to create your own pirate band from any of the characters (including the new ones taken from the Arabasta story arc). You'll sail across various maps searching for treasure, engaging in mini-games or other events, and destroying other crews.
Grand Adventure features 24 characters and over 50 support characters which can be used to aid you in battle. The game is in stores now.