I tried the TENS machine at our antenatal classes and really liked it, and considered hiring on.

Never got around to it, but when I was in labour i had all back labour, posterior baby, and could not stand ANYTHING at all on my back. Not a hand held massager, not someone rubbing my back, not ANYTHING!

I was so glad I didn't spend the money on one to hire cause I couldn't have used it.

In other words, i guess it would differ alot from one person to the next, and your body's sensations react very differently in labour!