I built up a bike specifically to be a beater-looking commuter (see my sig) but it's actually pretty nice and I've put a lot of work into it. Not a fixie (although I do have one, and it looks even more beater-ish).
I have a bolt-on rear wheel and Q/R front wheel and use a U-lock through the frame (around the downtube and the front wheel, and around whatever I'm locking the bike to) and have had no problems through my time at a major university.
Of course, I never leave the bike locked up on campus overnight.