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.