I've used a number of different brake systems over the last 25 years (with the exception of hydro disc brakes) and I find that the mechanical disc stop no better than linear pull or cantis (that are properly setup).

I do find that _quality_ disc brakes (in my experience) are better in frozen conditions and wet/mud. They are not affected by ice or water/mud like rim brakes. So in that respect, they are better. In the dry, no difference.

I have BB5_mtn on my commuter and BB7_mtn on my mtb bike. The BB7s are far superior than the BB5. I had Tektro disc brakes on another bike, and I felt like an oil tanker....I needed 2 miles to stop. Slight exaggeration there, but they were crap.

The BB5s were better than the Tektros, but a son of a gun to setup.
Like many I had to pre-tension the actuator arm for them to work properly. I have since discovered the business card trick to setup BB5 disc brakes, with no need to pretension the actuator arm.
http://www.ecovelo.info/2011/04/15/a...5-disc-brakes/
http://bikeshopgirl.com/2011/08/how-...id-bb5-brakes/

I am very happy with the BB7 brake system using XT levers. However, high quality rotors, cables and housing make all the difference. The rotors I have are XTR 6 inch. Unfortunately, the tab on the brake pad will strike the arm of the rotor, so I have to cut off the tab from the inboard brake pad.

There are advantages and disadvantages of rim and disc brakes. In a nutshell, I use disc brakes due to the muddy off-road riding conditons that would grind through a set of rims in one season. If you ride in dry conditions, then mechanical disc brakes would offer no advantage.

I can't comment on hydro brakes as I have no experience. But they come with yet another set of advantages and disadvantages.