When I was in college a few years ago, I only once rode my "good" mountain bike in. it's not that awesome, but it is much nicer than the average bike on campus. Usually I would ride a beater. Riding anything nice is just asking to have it stolen no matter what you have for a lock.

edit: my college bike choice was based on one thing. Can I replace it when it gets stolen, and how upset will I be at the loss of money.