I use it in the barn lot but would not put it in a stall. It's difficult to keep clean and not as soft to lie in as sawdust or straw. Plus, I would not want to force a horse to stand on it if they had to be stalled for any length of time.
We have a 12x16' area by our hay feeder that we enclosed with railroad ties, lined with ground cloth, and filled with pea gravel. While manure picks out easily, eventually, the dirt, hay, small manure pieces, and leaves pack into it, making it very dirty and turning it into a semisolid mass. If you don't shovel it out, rinse it, and put it back, eventually you have just pebbly ground.
-- There are two ways to slide easily through life -- to believe everything or to doubt everything. Both ways will save you from thinking.