I recently installed a front disc brake on my mtb commuter. I went with a shimano xt hydraulic based on reviews and because I found it for a good price ($110 for the brake, $50 for the ice-tech rotor). So far I have no complaints. It is lightyears better than the v-brake in the wet. As for stopping power, it is marginally better than my v-brake. There was nothing wrong with the v-brake though, and it performed as well as was required in every situation it was used.
If it came down to it on a new bike purchase, all things being equal between two bikes, I would opt for the disc. However, I would be more concerned about a bikes fit and ride as well as compatibility for racks/fenders and drivetrain before I worried about the brake type. In the city, with a lot of stop and go, I suppose the disc is nice. But, on longer treks on open road, I don't think I'd notice or really care which brake I was using.