Depends which TENS you are talking about. If it's the unit physios and osteopaths use, I couldn't comment as I have no experience of them.

However, if you're talking about the little units people use for pain management, they are the best! I have used one for back (chronic low back pain - 'nurse's back'!!) , shoulder (rotator cuff impingement) and post-op after a knee scope which helped me get back to work after just 1 week! I swear by them, have used them (occasionally!) for about 20 years, currently on my third unit!

These work by interrupting the neuro-transmitters that tell your brain you have pain. I always use 4 electrodes to get maximum effect and they work fantastically well. If you get one, make sure it has dual channels and several modes. It's good to change them every 10-15 minutes as the n/t get used to one mode much like the nose becomes accustomed to an aroma! They cost little - you can get good units quite cheaply in Lloyds Pharmacy, on-line or even on Ebay - and you can get on with life whilst using one. It's not recommended you use it whilst driving or sleeping, though.