I enjoy the platforming for what it is, and I'm a little biased since I grew up on 3D sonic, but bare with me. . In my head when I got this, I was expecting pure platforming goodness on one, hispeed action on the other. Inside, one is a platformer, and the other is....still mostly platforming, with ocassional switchs to 3D.
I feel that the platforming niche of this game is well filled by classic sonic, it seems sorta of silly being forced to play two "different" characters with very simular gameplay on slightly different stages twice. They should either be different enough to warrant their own stages to begin with (ala sonic adventures), or share the stages despite their minor or aesthic differences (ala mario/luigi).
Also, it's an arupt change of pace....for me it kinda kills things. One minute I'm barrling down the path through loops, grinding rails, and knocking over anything that dares to stand in my path....the next I'm slowed to crawl navigating the labyrith of platforms put before me. The pace should remain roughly the same or transision a bit smoother I think, it feels like I'm running into a brick wall.