I bought the FSR used, it was the RIM version and did not have disc brakes. I had already used the MP5 so I knew how effective the regen brakes were. Also, as far as disc brakes go, I have read that if used in conjunction with regen brakes, they might cause too much stopping power and force the wheel out of the drop set.
Something to consider- I have been riding mostly on the street, and the regen brakes work great. However, I went to the Alabama Hills and did some real mountain biking, and I find the regen brakes to be too much. When you are going downhill and you want to gently brake, the regen brakes overdo it and cause you to lose your balance. If I want to continue to use this bike as an off-raod mountain bike, I think it would be wise to get a rear disc brake. The beat of both worlds is to have the Bluetooth, so you can shut it off and on from your bluetooth device. I shot some video I will edit and post later.