I feel the opposite. I don't really care for the old Sonic games, but I really enjoyed Sonic Adventure 1 and 2.

Sure the idea of rolling around a 2D screen at the speed of light sounds cool, but factor in the pits they throw in the games and the entire concept is spoiled by the player inching forward instead to avoid all of the constant pitfalls and hazards. What's the point of going so fast when you know you're going to be penalized for it? That's what I found myself doing quite frequently in the early sonic games (the game gear version was especially guilty of this).

Sonic Adventure 1 and 2 also have this problem, but it isn't as bad. The levels are set up a bit more carefully, and because it is in 3D, you can actually see far enough in front of you to know you aren't heading straight for some spikes or a bottomless pit. I really also liked the way your bouncing attacks were homing, making it easier to make jumps in the process.

I also really liked the boss battles in Sonic Adventure 2, and to some extent, Sonic Adventure. I haven't played Heroes yet, but if I have the time some day I just might.