I can't remember which brand I had, so I might not be much help in that part. I bought it after having some traction and finding that to be helpful. I have a herniated disc (and have for years), and that gives me nerve issues and other pain. The theory is that the traction pulls you a little and eventually that disc "sucks" back into place.
So I spend a few hundred bucks on a table and give it a shot. I tried this fairly religiously for quite a while. I followed the instructions of not hanging upside down for too long and found it did very little to help correct things. I have a friend who is a massage therapist and got some advice from them as well. I tried different postures and things like that to try to improve the results, but to no real avail.
Eventually it became a glorified clothes hanger and I sold it. I guess my overall impression was that this could work for some people with specific symptoms or issues, but it basically did nothing for me....aside from give me a different view of the world for 5-10 minutes a day