Christ, what a topic.
I love co-op. It's the only online gametype I actually care about, since the other modes just aren't something I can appreciate in a Sonic game, I don't have a mouse, and focused competition in games drives me mad.
With co-op, it's the single player campaign (which traditionally serves as the main draw and emphasis for Sonic games), open for others to join. Having people around can spice up what would otherwise just be yet another runthrough, since we can take separate paths, choose different characters, comment on whatever, and see how things work out in the end. Once I'm done, I can use F12 to watch someone else play and see how they handle the stage. I can steer them in the right direction, point out shortcuts and tokens, or simply chat while I'm waiting for everyone to finish.
The presence of multiple people is a simple change that goes a long way in enhancing and switching up the experience of main game, without deviating too far from it. Granted, some people can be irresponsible or poor hosts, by refusing to set all players for exit to all, going hogwild on unnecessary cheats, etc., but that's why I typically just make my own server. The game's fun to begin with, and hey, if some people happen to join, then that'll just make things more interesting.
Funny thing is, co-op servers are rare for me. Usually people are playing match or ctf, or they switch out of co-op shortly after I join.