Xena here. My fork lock broke early on, so I needed something. Like the others say, it's only a deterrent. A bright yellow deterrent with a noisy alarm. I always carry it with me, but I don't use it enough (a bad habit, I know).
I've always been fascinated with locks, so at any given time, I've got 3 or 4 good ones with me. I have been known to use more than one if I'm going to be around a high theft area and away from the bike for any length of time. Bike shows/events worry me the most.
If you go with a disc lock, be sure to measure the pin size to make sure it'll fit.
