Yea, It's looking like you jump from one to the other, which is slightly disappointing. I would have liked the option to play 2D only, 3D Sonic has never close to matching the playability of the best 2D games.
 
I want the 3DS version, the graphics should look awesome.  
 
 
 

 
  
 

Graphically, the game looks amazing.  Now lets hope the gameplay holds up.

sonic needs pants.

Game looks gorgeous, like all of the best parts of unleashed. (there were good bits amongst the shit) As long as the stages don't get ridiculously hard and force you to restart from the beginning because of some annoying slow platforming death-spike section it could possibly be awesome.  The only thing I'm having issues with is the "Hey look we made what you wanted.... just like the old ones"  like Sonic 4 never happened. Colours  is awesome and seemed like a brilliant direction with some new ideas and it's a shame it looks like they have  ditched that for nostalgia.

I think they should have just stuck with one direction throughout the entire game. It's a bit jarring to go from old style to new style....I should know since I played it. The best way to do this would have been to make 2 separate games and put them on the same disc, so you could get to choose which style and still call it Sonic Generations. But from the demo I played, it doesn't look like that's the direction. I haven't read enough about the game to know if there's a choice or not though.