Disc brakes have more stopping power (if you have a wide enough tire), they work better in the rain, and they don't grind road stuff into the rim of your wheel when you stop. They also work better if you have freezing rain or water, as the rim of your wheel can get iced up and stop rim brakes from working.

Problem is, they sometimes make taking the wheels on and off more annoying (the wheel getting back on at a slightly different angle is more likely to cause brake rub with disc brakes than with rim brakes), and if they get wet they can start squealing like a banshee.

I live in Minnesota and have (hydraulic) disc brakes on my winter bike for the above reasons and they work great. No problems with them working in the cold.. But on my summer commuter I prefer rim brakes, despite their reduced performance in the wet, because I got tired of the screeching from my disc brakes. My dad just sold his disc brake bike because he found it so annoying (he also is always taking off the front wheel to put it into and out of his car).