There's some room for improvement, but it's still a great game that One Piece fans will get a kick out of.
Instead of (roughly) going through the plot of the manga and anime series like the previous One Piece games released in the US, Unlimited Adventure has its own original story. It begins when captain Monkey D. Luffy and his crew find a mysterious orb while at sea. Soon after, an island appears out of nowhere, and the crew soon discovers that there's something mysterious going on here. It feels just like the kind of story that would be in the series proper.
Oh yeah, speaking of the series, I should point out that this game takes place farther in the One PIece storyline than any US media has gotten so far right now. So if you're up to date on either the manga or anime, you'll be okay, but otherwise prepare for spoilers.
The adventure mode is the bulk of the game. Here, you'll be able to switch between the eight members of the Straw Hat crew as you explore the many environments of the island. Each character has their own fighting style to take on the many enemy encounters. For example, Zoro has his quick sword attacks, Sanji fights exclusively with close-range kicks, and Usopp fires projectiles with his slingshot. There's a character to suit whatever fighting style you may like, but it's important to give everyone some screen time. As the characters fight, their attacks level up and they unlock new ones. While you might like to stick with one character, though, it's important to keep leveling everyone's moves up, since if one character falls, you have to switch to another. You don't want to have just one good fighter, then lose him and have to try and scrape by with underleveled characters, especially during a boss fight. Also, the moves gain experience rather slowly as they get higher in levels, so you'll have to do a lot of fighting to keep up with the enemies, but everyone's weird moves are so neat and fun to use, it offsets the repetitiveness of the grinding.
The adventuring, though, doesn't really have much to counter its repetitiveness. In order to progress through the game, you have to open up passages by using that mysterious orb at certain spots to clear away obstacles. You can't just use it right away, though – You have to power it up first by converting the items you find into Orb Points. Furthermore, certain activation points require you to have specific items in your inventory, and not all of them are easy to obtain. As you get further in the game, you'll need more points and rarer items, so it's kind of annoying when your adventuring comes to a screeching halt because you have to go back and find something.
Some of these spots require you to make certain new items out of the stuff you find on the island, and the only place you can do this in the camp in the island's hub area. Here, you can make all kinds of things: Food, medicine, weapons, tools, and attack items. However, collecting the ingredients needed to make this stuff gets tedious, particularly when the ingredients for one item have to be collected from multiple areas. Also, this is the only spot in the game where you can save, so that's more backtracking right there when you want to prepare for a boss.
Said boss fights pit you against characters from the series, like Smoker and Crocodile, with the explanation that the island is reacting to the Straw Hats' memories and creating guardians in the form of these characters. These battles are intense, and even if you level up all your attacks all the way, some of the later bosses will still be challenging enough to take out half your crew before you beat them. Beating the bosses unlocks them for the game's versus mode, which has two parts. The first is the Team Battle, where each player creates a team of characters to fight with. Each character has a point value, and you have a limited number to work with, so you can have a big team of minor enemies and other low-point characters, or a small team of high-point powerhouses like Mihawk and Lucci. The second part is the Survival mode, a time trial where you mow down waves of enemies as fast as you can with one character, but it doesn't quite have the same level of fun and intensity as the Team Battle.
In a step up from previous US One Piece games, the characters no longer have that super-deformed look, and are now properly proportioned. They're also cel shaded in a way reminiscent of Wind Waker. And the characters have their voices from the FUNimation dub, so you can actually listen to them without wanting to mute the TV. No more always-yelling Luffy or Barney-the-dinosaur-with-a-cold-sounding Sanji. FUNi's voices suit the characters much better than 4Kids', though there were one or two times I felt the acting could've been better. The music is also pretty good, but the environments, though pretty, have sort of a "stock" feel.
The control handles well, outside of some issues with the motion controls. There's no problems with motion control in combat, since the attacks that use it need only a mere flick of the remote, but there are tools that require motion control – the pickaxe, butterfly net, and fishing rod – and they can be a hassle sometimes, particularly the pickaxe and net. It seems like they won't respond unless you make a big swing with the remote. Even then, it sometimes confuses a vertical swing with a horizontal and vice versa, and when you're trying to catch bugs, a wrong move like this can result in your target getting scared off like in Animal Crossing. While the controls for fishing work okay, it's still annoying when you're trying to find a certain rare fish. You'd think casting out farther would increase your chances of hooking a big or rare fish, but no. You can cast out as far as possible and still get a puny clam or shrimp.
The game took me at least 60 hours to complete, though a significant amount of that time was spent level-grinding and item-hunting. Your mileage may vary. Even after you beat the game, though, there's some additional play time to be had from the optional bosses, unlocking new costumes (Usopp's hidden costume = best unlockable ever), or trying to find all the fish and bugs, and the Team Battle mode is always fun to play.
One Piece: Unlimited Adventure is really a game for the fans. They'll get the most appreciation out of this game, from the short movies of scenes from the series recreated in 3D to the Straw Hats just being themselves at the campsite (Luffy eating meat, Franky playing his guitar, Sanji and Zoro fighting). If the considerable backtracking had been taken care of, this could've been up there with the best of the Wii's games, but it's still pretty enjoyable as it is, and one of the best anime-based games ever.