I commuted for a couple of years on a bike with canti rim brakes. When dry, they were awesome. When wet, they were good, but due to the oil/gunk that gets onto the rims when riding in the wet, it could take a few rotations of the tire for the brakes to become really effective and give me full stopping power.
Using a brake pad formulated for wet conditions helps with wet stopping tremendously (I used Koolstop Salmons). But even the Koolstop pads took a few wheel rotations to give full braking power, and I have had a couple of close calls over the past year because of the effectivness delay.
Earlier this year I built a new commuter that sports Avid BB7 discs. When dry, the stopping power is no different than rim brakes, but when wet I get almost immediate and full stopping power. I also find it easier to modulate the stopping power with the disc brakes, but that may just be a personal preference thing. I could not be happier with the discs, and don't notice the difference in weight.
I commute year-round, and it's wet here a lot in the winter, so wet stopping power is important to me. YMMV.