I used a TENS in labor with my first baby. It worked for me well, until I hit transition and forgot to use it. But by then, I was so "in the zone" I could have cared less about the darn TENS, and even if I had, I'm not sure if it would have made a big difference. I had a pretty fast transition, and didn't feel out of control at all. I was able to have a completely un-medicated labor/delivery.
I don't know what the current models are like, but in 1994, you had to manually dial it up and down with each contraction.