I played Mario 3DS at E3 and overall I really enjoyed it. They took the best things about the 2D side scrollers and the best things about the 3D games and made it into one Mario game. I have a few thoughts as far as some of the concerns addressed in the thread:
While its not clear that the raccoon leaf/tanooki suit can allow Mario to fly in this one, there really weren't any areas in the stages demoed to give him enough of a running start to test that. That being said I do hope that can be an option in other levels.
The fire flower can destroy bricks like in the 3D games which I really liked.
I'm not sure if the d pad even works in this game as I didn't even think to try it, but I can see where I would sometimes prefer it.
You have to be careful in that even when Mario is moving left to right, it's still a 3D plane. I died once while moving across a bridge because I moved the circle pad a little too far up while trying to move across and he walked right off. It could also be me though, since I've always had slight issues with games on a 3D plane.
Overall it was one of my favorite things I played at E3 and will be picking it up. It seems to provide a neat combination of the side scroller and 3D mario game.