I've heard of them.. while I'm a big supporter of using dry and live rock together, be sure to calculate the price per pound including shipping before you buy. Some of that fiji rock is kind of expensive, and if the shipping is a lot, you might as well get legitimate fiji live rock instead of dry fiji rock.
Price isn't the only consideration; you also have to take into account that the dry rock will not have all of the unwanted hitchhikers that are prevalent in fresh live rock.
Lots of people use Marco rocks and I've never heard anything bad about them. I use the dry Marco rocks myself and found them to be a great value and interesting shapes, good sizes, etc. I have about 50% marco rocks and 50% Fiji liverock from LFS.
I love Marco. However, I have noticed that my last purchase may be a little phosphate laden. I'd suggest muriatic and bleach soaks for a while before putting in your tank.