I've got one, used it, but I can't say it made any difference.
My friend tried it and ended up in a lot of pain....she wasn't unfit or anything either.
Took her a while to get over it, so I tend to eye it with suspicion now!
Me too!! Made my back worse!! Used it exactly as described, it was so uncomfortable to lay on, persevered as thought it might take a few tries but it felt like it had put out a disc or something had popped out! Had backache for ages after using it, had to do stretching exercises a lot to try to get whatever it was that felt wrong, back in place....... if you see what I mean. Just my experience, you might love it. Hope so.