I use mechanical discs, and find them pretty good. More stoppping power than v-brkaes, but they still have the cable stretch issues present in any other cable-driven brake. Hydraulic discs dont have that stretch, so you get a bit more precise braking from them, but all in all if your going to use it everyday, mechanical discs are fine...plus it lets you use standard cables, so if anyhting goes wrong, you KNOW you can just walk right into a bike shop and get the part you need, no waiting for parts to arrive.

disc brakes when wet do take a bit longer to stop, but once they bite, they bite pretty hard. If you ever had your wheels go udner water then tried to stop, you would notice they dont grab for a while either.

If you going to commute with a disc, consider a cyclocross style bike. The wider tires will make up for the stiffer frame and fork...but you pay for it in terms of efficiency, but not near as much as using a mountain bike.