HansRey - I love my Fatty and have yet to find anyone who rides it who doesn't come off with a smile on their face.
I bought the frameset and then set it up 9 speed with some shifters and mech I had lying around and a wide range 9 speed cassette. Standard stem, post and saddle I had lying around and some new Formula RX brakes direct from Formula (£130 all in!) with a 180 rear and 203 front disc. Got some wide carbon bars and a Truvative Holtzfeller chainset. The whole build was about £750. I have also drilled the rims and changed the insanely heavy tubes (600g a piece!) for some Schwalbe DH tubes to bring the weight down to about 28lb.
The bike is crazy fast on technical stuff. I am quite a light rider and once used to the feeling on corners it just rails everything. On technical or loose climbs the grip makes it climb well. Shallow fire roads are not great, but keep it at speed and it's not too bad. Tyre pressure is critical though, 7psi is about right for me.
I built it up to use up some spares, half expecting to sell it - that is not going to happen!