True. I haven't used it, but have always wanted to. I've been switching to tractors because of how wet it gets here, so haven't gotten the chance. I've read about it, and love the idea.
I'd imagine that it would be difficult in a wet climate. I guess that depends on OP's location.
I vote sand. I love the concept. Simply go in daily, or every other day and scoop the poop off the top. Simple and easy. Keeps it clean. Wood chips or similar you can really scoop the poop while still leaving the substrate behind. On a cleaning stand point, sand is pretty darned simple.