I've never used a TENS on my back, but have used one extensively on my wrist and forearm because of deteriorated triangular fibrocartilage.   I have a consumer unit that I got online about 10 years ago.  I also spent months going to twice-weekly outpatient occupational therapy where part of the therapy was a session with a big professional TENS machine.   

The pro unit at the clinic was definitely stronger and more beneficial in brief sessions.  But the consumer one at home is and has been effective with more frequent and longer sessions.  The key for me has been using quality pads.  At least with my machine, all pads (electrodes) are not created equal.  Some seem to get a better "connection" with your nerves than others.  My opinion is that the consumer unit is a good investment and once you find a source of good pads, buy a bunch of them.