Sez I used TENS for all 3 births and had 3 different experiences with it...

For Masons birth I found it to be a great help. It helped to distract me and I did learn how to time it so that it took the edge off. Once the contractions got really strong I laboured in the bath so it had to come off where I changed to gas.

For Angus' birth it was great in the early stages but then the hand controller broke so I had some dramas. One thing I noticed was the intensity would almost floor me when it would cut out mid contraction. It went from hurty but bearable to OMG, then bearable again as it cut in and out. I just twiddled the dials after that but it was a PIA! I took it off and got in the bath again.

For Calebs birth, it could have been the nature of that labour but it seemed that every time I put the TENS on my contractions would stop.
This labour was hard and fast once it got going so the TENS really did nothing for me but it was a security thing. I think it did relieve a bit in the early stages but I could have done without it.