No scientific evidence that it will do much of anything other than provide a very temporary relief from certain pains, basically if I smash your toe with a hammer, temporarily you;ll forget about and not feel your headache, For that you get to risk all kinds of nerve damage and infections.
I have not tried it on horses, but I know for a fact that it does work on people. I tried it as a last resort when I discovered that I had an incurable condition. I didn't put much stock into it at first, but it DOES work. Without it I would be chronically uncomfortable forever. It is not a cure-all, but can help immensely when paired with other conventional treatments. Just like any specialist, make sure your equine accupuncturist has good credentials and what he/she says makes sense to you. Done properly, it is a lot less invasive then conventional medicine, and should not be uncomfortable for your horse.
And all this is coming from a very scientific person who thought that it was all a bunch of bunk before being forced to try it. Now, I even considered becoming an equine accupuncturist after my life-changing experience with it.
I've had great success with it. My stallion had a neck injury at C-2 as a 2 year old. There was gait asymetry at the walk and trot. At the canter, he could only hop with the back end. In 1 session, he had an almost normal 3 beat canter and with 3, I was back working him. He does need a maintenance session about once a year. Weither or not you get any results from it will depend upon what you're trying to fix as well as the technique of the person doing it. My guy does best with the deep electrode stimulation over the straight needles. Probably has more to do with his size and mass than anything else.
I have plenty of friends who've used it with equally good results on breeding stallions, hunters and pleasure mounts.
I decided to give it a try. There is something going on with our mare (she is off at the walk) and I want to help her if I can. It is less expensive than x-rays and I've already had 3 vets tell me not to worry too much about it unless I want to start exploring (ie - x-rays, mri's, etc.). I made an appointment for next week so we'll see how it goes. I'll post after the treatment and tell how it went.