Wondering the same thing myself, from what I heard the first one was pretty dead soon after release.
The problem with the first game was it was a full retail $60 release so no one bought it until it was too late. Prejudice doesn't quite have the same problem, since it launched at $15, but from what I gathered the community is still pretty small from the bit I played of it. Sales like this bump up the player base enough that it will be worth it if you jump in relatively soon as the multiplayer that is there is actually pretty good! But don't expect to find a thriving community if you wait 2 or 3 months to get around to playing it.
Is Sonic 3 and Knuckles a good port on steam?
It's pretty much a straight ROM dump wrapped in an emulator shell with quicksave and whatnot, so.. yes?