I have installed the UUC kit in my Z8, and I am very very happy with it.
I have read all the loves and hates on the other boards, but I have to say that my experience is very favorable. The box itself is a little notchy, but it is much improved by the short throw, and added shift inertia provided by the heavy UUC steel gear knob.
It really does 'ease' thru the box now, and is an absolute joy at the track, as well as on brisk back road runs. Around town only the 1st>2nd is still seems a bit notchy, other than that I would say the 'sports feel' of the kit really adds to the driving pleasure of the car.
Mark Norris at BMW Independent Workshop (818 346 9363) fitted the kit, he is an official UUC dealer, and has fitted many of them into M5's M3 etc. It seems that the problems that people have had may well come down to the skill level of the person fitting the kit, as I have a friend with an M5 who has also had it insatlled by Mark, and loves it as much as I do.