I've been running the disc front/V-brake rear combo for a couple of years on my Surly. Works great. 70%+ stopping power is in the front brake, so you get big benefits for half the cost of a full disc set up. Plus, I like to run fixed/free hubs in the rear of my mountain bikes. The only time I have felt a need for the rear disc was in icy conditions (below freezing + lots of streams around here = iced rims), but that's solved by running fixed.