my dad used to have a pair and used them for his lower back.
He said they were great but a pain in the ass to get in and out of position.
Ive had back problems on and off for year and have thought about an inversion device as well.
Almost all of the serious power-lifters I knew from my old local Gold's would use either the gravity boots, inversion table, the Loui Simmons-style reverse hyper bench, or any combination of those.
While I can't be 100% sure, I'd say that a big reason the reverse hyper saw the most use is that many of those lifters were bigger
(and sometimes fatter) guys who weren't the most graceful of homosapiens. Many of them looked like they'd have a tough time getting into position with the boots, and I doubt that our poor table could have supported their...um...mass.
But, they all touted the benefits of decompression offered by this equipment.
I wish every gym had one of these:
