I'm finishing up my undergrad here right now. I admit, I don't know much about the bus system, 'cause my crippled butt ain't walking to the bus stop, but if it's as bad as Greenville's (my hometown), then it's pretty bad. The bus is pretty much it for public transportation, too. We don't go for light rails and subways so much down here. (With the exception of major metropolises like Atlanta, of course.) USC does run a campus bus system, though.
My little sister did her undergrad at USC, too, and she doesn't drive. As long as you live within walking distance of campus, you're also within walking distance of the whole 5 Points/Vista/downtown area, so I guess it's not so bad. I should note that my sister's idea of "walking distance" is a bit more generous than mine.