My fair weather commuter has a set of Paul Racer Medium rim brakes, which are AMAZING. I love them.

My daily commuter has Avid BB7 with 180mm rotors, which I also love. After swapping several rotors due to unacceptable truing issues, I have found my Shimano XTR rotors to run perfectly true.
I like the mechanical Discs because they are extremely easy to work on, and once set up properly (also pretty simple) they work extremely well in all weather conditions. My brakes work equally well in wet or dry, and are much cleaner than their rim-based counterparts. If I didn't contend with frequent rain and snow, it would not matter as much to me, but I find I prefer the discs. Maintenance is easier, too, with the mechanical disc brakes, than with rim brakes. I also like the fact that I am not wearing my rims down. My disc-based wheels should outlast an equivalent set of conventional rim brake wheels given proper use and upkeep.

My wife's bike has a set of the Shimano Alfine hydraulic discs, which work very well but the added hydraulic aspect does not excite me. I've been reassured by those who know more about hydros that it is no big deal, and I'm sure I'll find out for myself whenever it will be time for me to work on them (I do all of my own wrenching).