I have a Gaim shoulder/head stand helper and, after coming across this thread, I started doing up to a minute or two every other day. My hair looks better but I kind of think it may be attributable to other things that I started doing simultaneously (e.g., taking Arnica, regular tea tree oils, mega D3 etc.) Anyway, wanted to share that I agree totally with comment below re relaxing effect and add that there's an obvious benefit to health -- feel like I have had a 45 minute massage after I invert. Relieves all the trapped energy in spine/hips, waist looks longer, just feel/look "better" - hard to describe. Just try it and don't give up, regardless of hair effect -- remember, hair follows health!
Inversion therapy is good for your health, not just hair growth. I did the headstand using my body lift for 5 minutes, 30 days straight. I don't know how much growth I got, but my hair was full and thicker. I prefer my slantboard which is basically the same thing, inverting your body. I lay on that thing for 10-15 minutes, 5 days a week. It's so relaxing. I try to do it first thing in the morning, or before I go to bed. This does contribute to fast hair growth because you're getting the blood to the scalp.