Well he is almost everywhere. His merchandise can be seen some local toy stores... well, not anymore. People make Sonic-based fan sites, webrings, and possibly fan arts.
Yes, Mario is Sonic's rival... But it wouldn't last long since Sega became a third party team. So now since Sonic is now in certain consoles like Nintendo Gamecube, Wii, XBOX 360, and Playstation 2 (and 3).
And let's not forget, Sonic and Mario is now going to be at the Olympics for the Wii and Nintendo DS.