I had TENS treatment briefly for back pain a few years ago. It didn't do anything for me then, and I can't imagine that the stimulation would be all that useful during labor unless there was a way to apply it directly to the uterus.
I discussed it with my OB for my first but then didn't follow through on it. I saw it mentioned recently on baby center or somewhere which reminded me it was an optiong but I don't know anyone who has used it.
They talked about it at my Doula training, but mentioned if we wanted to use it on our clients we had to provide it, not the hospital. They said it can be helpful in labor, esp with back labor. however I have not had any experience with it.
Re: TENS