I use a Kryptonite U-Lock which was given to me many years ago. While I don't keep my bike in one place all the time, the use of the lock has prevented someone who just wanted to take it and ride off.
No one here goes looking for bikes with mechanical tools, hacksaws, or blow torches, all of which are tools to cut through the lock. But I don't kid myself, either. If a theif wants a bike, he or she will get it.

So: How good is a Kryptonite U-Lock?