I was at a barn, years ago that used it. The advantage, for them, was that it's cheap, cheap, cheap. Would I ever use it for my horses? No. It's been so long that I can't remember all the reasons why, except that I recall the empty hulls fill with urine, and they aren't as comfortable for horses to lay down in, or as absorbant as shavings. If they aren't cleaned soon enough, your horse will be standing in pee.
I attended a couple clinics at a barn that used rice hulls, and I wasn't a fan. Perhaps because it was something new to them, my horses nibbled at the bedding. The pee wasn't as easy to pick up. And though it may have had nothing to do with the bedding, that particular barn was absolutely infested with mice.
"I did know once, only I've sort of forgotten." - Winnie the Pooh
We use it exclusively in our barn. I love it. It's super deep and supportive so all the horses love it. I would say we have 6-9 inches of rice bedding in each stall--it does not break down like shavings. When we switched from shavings, my horse would lay down for hours. The only drawback is it's not absorbent so you do have to scoop out the pee daily but it's super easy to screen out the manure. We have no urine smell in our barn and no flies. Several vendors use an enzyme now to help with the urine