Hi cfreund,
Yes, I used a TENS unit as part of my TMJ treatment and had fairly good success with it. Mine was somewhat different than the one you described, but the intended effect was the same. When I was going for Physiotherapy, the type of TENS unit my physiotherapist used on me was for "Microcurrent Stimulation of Accupressure Points". I usually got some immediate relief from my tight muscles, and muscles in spasm. Eventually, I got sustained relief that lasted for days. I finally reached a point in my physiotherapy that I purchased a TENS unit intended for home use (also somewhat different than what you described, and smaller). It worked on the same principle as what was used on me by my physiotherapist. At first, I used it quite frequently, then only on occasion. Can't remember the last time I had to use it now! It cost about $150.00 (Canadian).
Hope this info helps,
Marlene