The jump to 3D gameplay was not kind at all for this (already kind of stagnating) franchise. The original Sonic Adventure games really don't hold up very well, Shadow the Hedgehog was unbelievably stupid, Sonic Unleashed only slightly less so, and Sonic 2006 was horrendously awful. The rest of the games, spin offs or not, aren't very good (or at best, mediocre) either with few exceptions. (Edit: Had to generalize a bit. How do you distinguish between a main game and a spin off - the line is pretty blurry.)
This is a franchise that has seriously overstayed its welcome. Goodwill from nostalgic fans is fine, but it can't be the only thing carrying an otherwise decrepit series.