Hey Sarah,
I was asking my sister this exact question yesterday! As you know I am thinking along the same lines as you re: pain!
She used one for her first birth. She was induced in the morning becasue of high blood pressure, and ended up having an epidural for the same reason later that afternoon/night. The baby was born around 10:30pm?
She said that she didn't think the TENS did all that much, but it was a very nice distraction during the 12 hours before the epidural! They hired one, and I think were trained by her physio to use it (remember me saying she had carpal tunnel and had physio as a result?)
I have read that some people SWEAR by them. I wondering if I should give it a whirl too?
Fi