That's one nice bit of coding!
The characters in general move and feel like their counterparts. I haven't played Advance2, but if you tried to mimic the original levels, you did great. If you just used the patterns, but designed that yourself, then your level design is excellent as well.
Main off points:
-Physics: Gravity should be a bit higer (character's feel like they're floating. Also Sonic should bounce of the badniks a bit more. Accelration (how long it takes to go from standing to full running) should be increased a bit.
-No grace period! After being hit by a badnik, there should be temporary invincibility.
-Also for Tails and Creme, it's difficult to use the springboards without activating their special moves.
Sorry if I seem picky, but yeah, I play Sonic too much. :-P. Nice work!