If it is the same stuff I have seen before, I would not put it in my SW tank. I believe that it's made from concrete. I used it in a FW setup years ago but thinks the use a concrete concoction in some type of mud mold to get the cavities in it.
I beleive Lace rock is natural forming. I have had a couple of huge peices in my tank for around 6 years now. It will get covered with Corraline algae and will have a ton of feather dusters attach to it (well at least in my tank it does) I dont think your could count on it for very much in the way of filtration, but it makes good base rock.
Lace rock is definitely nice for building, and like mojoreef's mine is covered in corraline and tiny feather dusters. I know the stuff I get is natural.
I know some people have experimented with rock made from concrete, which of course would mean you could make it any shape you want. From what I've read, if you don't cure and clean it right it can really cause problems. That may be what DonW has seen happen.
Lace rock is safe. Sometime ago someone sent some to Dr. Ron on Reef Central for him to check out, and according to him it is safe in saltwater. I've had some in my tank for years without any known issues from it.
It's only downside is it is more dense than liverock so you add a lot more weight per piece of rock than you would with liverock, so IMO the lacerock isn't nearly as good for denitrification.