I really liked the first game in the series, but I didn't know how they'd do a sequel. Now I know. Their secret was to put an accelerometer and vibrator in the game cartridge and make most of the games involve rotating your Game Boy around. This sounds more complicated than it is; in my GBA SP, the twisting motion is handled very well, and the control is dead-on for most of the games. The challenges include aiming the beam of a lighthouse to illuminate a raft, balancing a stick on a moving platform, pumping a railroad car to escape a boulder, and aiming a superhero to fire fingers into giant noses. I've only gone through about half of the story mode so far, probably unlocking less than half of the games. In story mode, you play a series of games with each character to unlock boss modes and minigames. Then, you can the games separately to get high scores which will unlock more objects. I've already unlocked a record player toy where you have to spin around the GBA to keep the record playing, and the speed at which you spin the device determines the playback speed.
Fry's Electronics has it for $25 this weekend; their normal price is $30. If you like silliness and have a GBA, go pick it up, and I'm sure you'll have a blast.