Wow, everyone must be taking the evening off.... I'll give you my idea; I've seen this stuff in my area on 50's/60's construction. It basically looks like coarse stucco. It's put on over plywood or similar, likely applied like stucco only with pebbles instead of fines.

The stuff I've seen holds up very well unless, like anything, it gets water behind it. I don't think it was very common, at least not in my area, but when I do run into it, it's usually in good shape. Like any other exterior covering, you're most likely to have a problem at a seam or penetration.