Great, but could be done better.
Definitely a well made flash game.
You could have done better on the gameplay and moves, though.
Jumps felt a little too loose, movement didn't feel firm enough, very limited attacks/moves, etc...
Some more complex moves could have been used as well, but I did enjoy Knuckles throwing huge stones at goombas. XD
Basically, I'd say if you do a sequel make the characters feel a little more like they do in their respective games (where applicable.) Like giving Knuckles the ability to glide and climb walls, do a 3(?) punch combo, etc...Megaman's cannon would make more sense if the basic attack was like it is in the games: pathetic. Then the charged shot should have more damage/have some knockback.
These are just suggestions, though...hope the feedback is taken well. ^^
Still good, though.