I’ve just installed some 7″ disc brakes. I had a thought that maybe I would like to get the hydraulic brakes, that uses hydraulic pressure as opposed to cables, but opted to get the Avid BB7 mechanical disc brakes and would recommend them to anyone. They are very efficient brakes in the dry or wet weather.
One recommendation from the bike dealer was the importance of keeping the discs clean. Often things like grease and oil can get on the disc and get on the brake pad and it can be very difficult to clean. So each ride I wipe down my brake disc when I get home. It takes two seconds (margin of error of 1 minute) with a clean cloth and some plain-jane Isopropyl alcohol.