Even for someone who doesn't even read the manga series can take a part of the game and enjoy it!
In the beginning, I didn't understand a thing! Have never read the manga, so I weren't familiar with the main characters. For an example; That Ruffy guy could inflate and stretch his body parts. Well well... enough with my babbling, let's go on to the good and bad things in the game:
The good things:
This game is full of unexpected events, lots of fighting with all nine characters that you could switch at any time, a bunch of items that you later on can build tools with (like the vacuum cleaner and the fishing rod).
One cool thing that made me play this game hour after hour were the "level up system".
Pick out an favorite character, fight a lot with him/her and he/she will get a bunch of extra attacks and combos. If you like an specific combo or an special attack, you can upgrade it by using that attack more than usual.
The bosses in the game is always a challenge, at least for moderate player like me. The bosses are creative and uses powerful special attacks when you think you're about to defeat them. That's a one piece boss in an nutshell! (literally speaking)
One last thing, the graphics of the game are pretty good to be a "toon" game.
The bad things:
At some places in the game, it fells like you're stuck. It's an item that you must have, but you can't find it! Hour after hour you search for it, but no a clue. Then you suddenly realize that the item you search for are on an completely other island or on the ship. That happened to me a couple of times. It's really frustrating.
Some last words: Even for someone how don't read the manga can enjoy the game! And those how read the manga, I think you wouldn't be disappointed.
(I think I should start reading the manga...)