I stopped playing Sonic after Sonic 3 & Knuckles (yeah, the combo cartridge), which, IMHO, maxed out the possibilities of the game. Also, the replay possibilities are so numerous I'm not even sure I'm done playing it!
I don't dig the episodic format, though: Sonic games are to be played FAST. Still, kudos for the 3D graphics adaptation: probably the best scroller I've seen yet.