I'll be forthright. This is a fun little game. It makes use of the DS functionality quite well (Most games are stylus only, some use both stylus and buttons, and some are button only.)
You can play as a bunch of Mario & Sonic characters, and they all have their strengths and weaknesses. I've been using Tails mainly because he's got decent speed, good stamina, and he's got great 'Skill', which I'm not quite sure what that means.
There's also a wide variety of events in the game. I've gotten good at running & swimming, and most field sports outside of the Triple Jump, which I can't make work for some reason. That's my only gripe, that some of the games don't explain the rules very well, namely the Triple Jump. Also, Ping Pong and Shooting are kind of hard to perfect.
I rate it... a 3/5.
edit - changed score. It's worth a rental, but not to buy. Thanks everyone for pointing out my miscalculation :V.