I borrowed a TENS from a friend for my 1st labour. I really don't think it did much but was a bit of a distraction for a while. DD ws posterior, pain was too full on so I had the works (peth, epi) in the end.
My mother came in at one stage and put the thing on max accidently which felt like my back was tearing apart; you can imagine what words I used then LOL! #-o

With DS I didn't bother using one, my labour came on way too quickly and only lasted 2 hours so it was gas and that was it. I find that gentle rocking in a hot shower, massage, using a bean bag and floor mat, plus hot packs work just the same to help with back pain.

As someone mentioned I don't think they can be used in water which can restrict a little.

Good luck