I finally played through Sonic Generations.
The 2D sections are very well done. Not perfect, but close enough that it felt like playing the Megadrive Sonic games. I agree with Draikin that (at times) it felt like the gameplay became secondary to showing off the set pieces. Particularly in the later levels. Also the levels offered less exploration than Sonic 3 & Knuckles (no special stages to find, etc) which is a shame. Nevertheless I would be very happy to play future Sonic games based on this engine.
The 3D sections are another story. Unfortunately the major issues with the Sonic Unleashed gameplay carry over. Sonic is still slippery to control and the player is punished if a very precise movement path isn't followed.
It's a shame about the 3D sections, because they could be better. I still think that Sonic Adventure 1 and 2 had superior controls. As a test I went back and replayed Speed Highway and City Escape in those games (both levels were included in Generations, but the gameplay is closer to Sonic Unleashed). Even though the SA games are in some ways less polished than Generations, I remain convinced that by taking away control from the player, the Sonic Unleashed/Generations 3D gameplay is inferior to that of Sonic Adventure 1 and 2. Sonic 4 has a similar problem.
Another questionable aspect of Generations is the inclusion of a level from Sonic 2006. That game was so bad that it was removed from the Xbox Live marketplace. At least the level was remade, but I would much rather have seen homage to a better Sonic game instead. Perhaps Sonic R, which was quite groundbreaking for its time. They could have grouped the Saturn and Dreamcast levels together and moved the Sonic Heroes level over to a post-hardware-Sega section.
I haven't played through all of the bonus missions yet, but there are some gems. Such as a level where you must bounce a flag through the stage without hitting the ground, Sonic & Knuckles style. I'm glad this game was made even though I only liked half of it.