Matrix has gotten great feedback from lots of fishkeepers. It works fine for me, but I don't see what the hype is about. It looks just like the little pieces of driveway gravel my neighbor stuffs his canister with. According to this link it is pumice rock:
http://www.fnzas.org.nz/?p=1778 that is guaranteed free of silicates or heavy metal. The two liters I got contained lots of natural variation in size and some of the rocks were shiny smooth black rocks that looked like obsidian. Since those rocks didn't belong it made me question their quality control- some of the Matrix could easily have streaks of something ph-altering. The Matrix generates a lot more grit that Fluval Biomax, which is annoying, though luckily I think the grit is too heavy to get in impellers.
On the other hand it's a little cheaper than the sintered glass substrates (though not too much cheaper, if you get outsize and unusable pieces), and its variable pore size is supposed to be able to support anaerobic and aerobic bacteria (no idea if this is true).