The marketing info from Billy Bags has absolutely no information on what "Cracked Glass" is.
If it's similar to laminated or security glass it might possibly have some merit as a shelf material but still wouldn't be my first choice; beyond that, if it is basically a simple glass product I wouldn't have much use for it. Dogmatic or not, I have not heard a "simple" glass product that sounds even remotely decent as a shelf material.
I've experimented with granite, marble, glass (basic and laminated in various thicknesses), cutting boards, phenolic (which is pretty similar to the Black Diamond shelfs in performance at a fraction of the price) and acrylic and I agree with Lak that acrylic makes a very good shelf material.
Acrylic can, however, start to get a bit pricey, particularly if you need it to support a bit of weight.
An extremely cost effective way to create a great shelf is to put together a constrained layer using a thinner sheet of extruded acrylic on top, a piece of baltic birch (which is dirt cheap) underneath it, with a piece of thin (1/8" or so) neoprene sheet between the two. The neoprene sheet should be available online or from an upholsterer.
This is what I'm using under my table now with an older style Target wall mount and it is, by far, the best and most natural sounding shelf material I've ever used.
I'm using 1 1/4" phenolic in the rest of my floor mounted rack as shelf material (under integrated, phono preamp, etc) but I'd guess that the above would outperform it for a fraction of the price-it definitely did under my turntable.
