I have to ask, as I have a TENS unit for my bad back: how do you use it in play? It would be great to get more mileage out of the thing.

To try to answer the original question: I've been in pain (my back, but sometimes runs down my legs as far as the ankle) for almost 7 years. I have good days and bad days, but I am almost never pain-free. The pleasure/pain of bdsm is worlds away from that pain. I may have gotten the endorphin rush once or twice, but what I get more is the pain gate thing: if I can survive the back pain long enough to get excited/aroused, then the bdsm pain over-rides the back pain. I've gotten a few hours of reduced pain after, but more often, I end up with more back pain because of the positions/movements of play and sex aggravate the discs and muscles. I try to take a dose of pain meds right before we start (I've been on them these 6 1/2-7 years, and I don't get high off them anymore, so I can still be safe, sane, and consensual,) but taking the meds before sort of heads the pain off at the pass, so to speak.)
Good luck with your pain.