My partner went and bought one after I had a painful false labour a week before the real thing. Baby was back to back, but I managed to turn her in that week and the actual labour wasn't as painful!
I used the tens up until getting in the pool and it felt nice and tingly... I'm not sure if it did much but distract from the contractions. I kept cranking it up and my partner couldn't believe how high I had it come the end. He'd tried it on himself and couldn't get past the second setting without feeling it was too strong lol
So I think it's worth it even for the distraction - as Wednesday mentioned, for something to do. It's nice to have something you feel you can take charge of and control while contractions are happening.