If your DH feels they are a bit iffy, you can tell him that there is reliable scientific theory about how and why they work. However, you both have to feel comfortable about what ever labour management you decide on, you both have to be on the same page...
This is great advice for you, Liz, except I wonder how relevant it is that your DH is comfortable with such a non-invasive pain relief method?!? Surely if it works for you he should be happy with that? You're the one who's experiencing the pain after all And it isn't like a drug that's going to cross the placenta and affect your baby...
I've not used TENS in labour but have at the physio for other pains. Personally, I didn't find it relieved my pain as much as distracted me from it. But that could be a useful tool when the time comes too!
There's no harm in looking into it. GL!