I have one, and it works if you are in severe pain, as the pain the tens machine inflicts is greater than the back/muscle pain etc. I use mine on the "wave" setting, it slowly increases in power, then ebbs away, then back etc. I do find that it works for me, and after using it for an hour I feel much better for several hours, although i think the effect may be psycological, a different pain to take your mind of the primary pain.
Annie.