I had a TENS machine and was induced (although was in labour when they induced me) and it worked fine. Although I did keep forgetting to use it because I wasn't in that much pain.

I took it off to use the pool just before I started pushing and didn't put it on again afterwards and it still wasn't painful. I guess it didn't do much for me, but maybe it did - I had it on for about 6 hours while I went from 4cm to 10cm. (But then I was "oooh, that's a contraction, better press the button, ah it's finished now.") I know that having the constant pulsing buzzing there was really relaxing and better than nagging DH for a massage.

I used it again when I had really bad whiplash and backache and DS was 2 months old and I was in a lot of pain but couldn't ignore him: it worked wonders then!