When I had a stress fracture in my femoral neck last summer, I was given an ultrasound machine to help the bone heal. I recently read that ultrasound could also help with soft tissue injuries. I still have the machine, so I was going to dig it out and try it on my tendon that's been giving me issues since August.
I decided to do a little research on it before putting it to use, though. I didn't do any research when I first got the machine, but that was because I was in the middle of moving and leaving my job. I was busy and didn't really have great computer access. Now I find out that I could have saved some effort.
The theory is that ultrasound helps with pain and speeds healing. There have been studies that showed this to be true. Although the one I found was small and on rabbits. Another study showing it works involved people and was a bit larger, but was funded by the company that sells ultrasound healing devices. A study on rats showed promise, but only if used early after injury. A review of all ultrasound studies found bad science and no proof of effectiveness. I think the Wikipedia summed it up nicely: Effectiveness of therapeutic ultrasound for pain, musculoskeletal injuries, and soft tissue lesions remains questionable.
If I didn't have the machine already, I wouldn't go out and buy one. Since I already own it, and have an injury that MIGHT benefit from using the machine, I will use it. There's no side-effects from it, so I don't see why I shouldn't do everything I can to make my tendon feel better.