Walt Smith is indeed a man-made mix of sand, shell fragments, and concrete mix. More dense and less porous than natural rock like pukani, etc. I knew that before I ordered though. It has been pretty clearly described on Walt Smith's site as well as most (but not all) of the retailers who sell it dry ... as well as on the forums.
I also chose Walt Smith rock from TBS four years earlier, so I knew what to expect from it.
In my case, I was attempting to set up a Java Sea biotope and wanted as much as the life as possible in my tank to be native to that region. And I definitely want to go with live rock that was seeded in the ocean. Since no more rock is shipping from Indonesia though ... e.g. Java Sea area, the Walt Smith rock from Fiji was as close as I could get. (It's the only live rock still coming out of the Pacific that I know of.)
Not nearly as much life on it you'll find on TBS rock ... but still enough to make it interesting, and hold close to my biotope goal.
The only real draw back I've encountered with it ... other than it being a little heavier, is that having less micro-pores than natural live rock may reduce its ability to turn nitrate in to nitrogen gas. E.g. processes nitrate a little less easily/efficiently. But this can easily be compensated for by using a sand bed or something like zeolite, Seachem De*Nitrate pebbles, or a Brightwell brick.