I've had mountain bikes with v-brakes and with discs and have discs on my commuter/tourer, and there's no comparison for me. Hydraulic discs stop me much more quickly, stay dry and effective when I cross waterways, and the pads are far easier to change than bolt-on brake pads. They also don't wear out the walls of expensive rims, none of which are cheap to replace. I've heard that mechanical ones are better for touring in case something breaks while you're out in the boonies, but in thousands of miles on hydraulic discs I've yet to have a system leak or become otherwise non-functional. Like clipless pedals (SPD on my mountain and touring bikes, Look Keo on my road bike) I swear by them.