Just cruising the news and found an article from CNN on hwo TENS units don't really work.http://www.cnn.com/2009/HEALTH/12/30/back.pain.tens.nerve.stimulation/
I have a unit. I have to admit I just don't use it. I've tried but the hassle outweighs the benefits for me.
It did seem to take the pain level down a notch though. Maybe it's just that it feels good compared to just straight pain. The ultrasound unit at the PT places always felt MUCH better than the little TENS unit.