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.