Look at the following threads for more information:
I have a V-Brake on my KH29 because I pick up speed with it going down hills.
I used to have a V-Brake on my KH24 but took it off because I rarely used it when doing technical MUni. Now I am going downhilling old Garbage mounds. ( Usually about 30 to 60 meters high with a 60% incline!) I am thinking of putting a Magura on. Chain reaction bikes does have good prices.