It depends mostly on where you are and what the bike thieves tend to do. College campuses (campi?) are prone to lots of bike thefts, so you need something very effective.

As a general rule, you want to have one of the best locks compared with other bike commuters so that the thieves see your lock and move onto the next bike. Also, you want a less-desirable bike than average. If you like to own a nice bike, ride it on the weekends, and get a beater bike for commuting.

I had a hotshot racing bike in college. It was my pride and joy. I used a U lock, which was state of the art at the time. A thief dismantled the rack it was locked to and took a piece of the rack, with lock and bike. Probably tossed it in a van or pickup truck and then worked on the lock in the privacy of his garage. It hurt like hell. Don't let this happen to you. My roommate loaned me his beater bike, since he wasn't using it. I've owned a beater ever since then. Sometimes you get more attached to your beater than to your spiffy bike.