Well I took a ride today and I can confirm what mymindsok reports about the rear brake...it was pretty much worthless. On a positive note, the riding position felt great and the shifting was perfect, I have no complaints.
Mymindsok, did you have any of the install problems I had, especially concerning the shift linkage? I have a '76 R90/6. I wonder if there are variations in the frame or trans geometry.
Another thing about the rear brake..If you have an aftermarket sporty exhaust like I do (Luftmiester 2 into 1), you will notice the brake lever is very close to the exhaust. It doesn't touch, but my foot will touch the exhaust pipe if I really push on the pedal. The way the rearset is designed does not allow you to adjust the height of the brake pedal, but it could be modified to do so.