Customize your robo with a wide selection of parts, then battle. Really fun in both story and multiplayer modes, though the story is a bit predictable (I pegged the main villian the first time I saw him). It managed to conquer my DS from Pokemon (this is not an easy feat).
So, discuss the game here, maybe post your friend codes so we can play sometime. Mine's 4167 1274 6596. Username, well, I think you can guess that.
Oh, yeah, if you haven't, check out the ingame character profiles in story mode. Some of the likes and dislikes are rather amusing.