I got my first bike stolen in high school. It was a $20 piece of crap my dad bought off of an auction or something. I was very sad because $20 was a big deal to me back then, and the bike was actually way better than my mom's bike, which was a piece of $hiT.
But anyway, I never leave my bike unlocked. And if I have to leave it unattended, I always make sure either it's a populated area, or there's cameras somewhere watching my bike. Even if it's locked, most locked can be easily cut with a cable cutter. But then again, I've never commuted in big cities, so ymmv