I've never used one. But a regular rasp is easy to use and a whole lot cheaper. So quite frankly I don't know why there is even a market for the Riders Rasp.
The only advantage I could see is if you wanted to carry it with you riding. But I have been doing "barefoot" for about 5 years and never once have I gotten off my horse on a trail ride and felt like I had to have a rasp at that very moment.
So the only thing I can think of is that it is preying on the fear of horse owners that their horse's feet will somehow fall apart if they get chipped or something. But if you put a good roll on them once in a while they almost never chip, and if they do it's not the end of the world. It usually just means your horse was in need of a trim anyway.
So I guess I'm still trying to figure out the whole Riders Rasp phenomenon.