I have one. It helped me with sudden and unexpected acute back pain when I woke up one morning that went on for over two months, no relief sitting standing or sleeping... I did 10-15 minute inversions starting with the least inclination for a few sessions and then built up to full inversion about two weeks later. Amazing results for me, completely took care of the issue which i suspect was a misaligned vertebrae due to a compressed disc. I thought for sure I was headed for ongoing pain management interventions or surgery down the road. There are contraindications though, so not for everyone.
Tried them, but would get bad headaches and having glaucoma added to the problem.
The neurosurgeon that fused my neck suggested that I just hang from a pull up bar instead with chin over the bar. It stabilizes the shoulders and allows gravity to elongate the spin. Works great for me, 30 seconds into it and my back will crack from the middle all the way down to the lumbar. You get a little strength benefit at the same time.