As kiddy as the game is, you'll still find a lot to like.
The story is of this monkey named AiAi who lives on this island. Well, one day this evil monkey named Dr. Bad-Boon comes and steals all of the bananas. (Oh no!) So AiAi and his gang of four monkeys, Baby, Mee Mee and Gon Gon go on there little adventure to stop him.
The monkeys are inside balls. (Hence the Super Monkey Ball name) and you use the analog stick to move it around and collect bananas. You try to collect as many bananas as you can and make it to the flag at the end of each stage. Some stages can be very easy but others painstakingly tough. The game doesn't have too steep of a learning curve because it controls so simply but you're going to have to master the game to beat some of the later levels.
The rolling around in balls is the Story Mode portion of the game which has tons of levels and will probably take you a while to fully complete. The other part of this game is its minigames which focus on multiplayer and are very well done and fun to play. The minigames are Tennis, Bowling, Dog Fight, Race, Golf, Fight, Boat Race, Shot, Soccer, Baseball, Billiards, and Target. A lot of these play like the real sports. Some of these minigames kind of suck but most of them are very fun to play and control well. A blast for any party.
The graphics don't show all that much greatness although they are good. The graphics are very clear and the game has a smooth frame rate but there also isn't much too the graphics because the lack of detail. Better backgrounds wouldn't have been too much more to ask for, would they? As for the sound, it's terrible. The music is annoying and repetitive and after hearing the way these monkey's "speak" during their little dance, you may want to kill yourself. Also, the cutscenes are terrible and don't offer much apart from an awful dance that the monkeys do which is downright stupid and makes you wonder why they made cutscenes for this game.
Other than that rant, this is a great game and I suggest you try it out now that it's a Player's Choice title and has excellent multiplayer.