This looks quite cool. I like how they're making use of having two screens to extent the size of the environment instead of focusing so much on the touchscreen. When I first saw the Sonic demo back when the DS was first revealed I was really uninterested. But this looks like the Sonic game we've wanted for a while.
Only Nintendo can take a sidescrolling 2D game and turn it into a great 3D game. Castlevania, Sonic, Mega Man, Contra, etc should all stay 2D because it's clear the companies that own those franchises don't have the skill to change dimensions.