I have used similar devices (loaned or applied by my chiropractor) on muscle spasms or muscular type of injuries on several occasions.
I recently used one after tearing muscle fascia on my left quadricep as part of the rehab. The device works well as a form of deep muscular massage, but as some are marketing as muscular stimulation my chiropractor says it doesn't work that well.
Unless you intend to use is it as part of rehab it would be best to save your money - IMO.
I used to work at a division of Medtronic that sold and leased out TENS units as part of pain management. They were helpful in treating people who had chronic pain who were not willing or wanting to rely on drugs. They worked on percentage of the people who used them but not all people found them helpful.
I tried them out a few times but didn't seem to get anything out of them. Back then I didn't have anything hurting though. For me the sensation was that my skin was burning. It was very uncomfortable.