I haven't posted on here in a while (school, moving, and a new job are keeping me busy)...but I love my TBS rock. I've had mine for almost two years now. I never had a mantis make it into the tank. I had a few gorillas (and probably still have a few gorillas). But I got a very cool pistol shrimp and a TON of porcelain crabs. The serpent stars are amazing to watch.
I'm with Dennis on the density of the rock. It doesn't matter to me in the least. My rock has tons of interesting shapes and lots of holes. The majority of the corals that came on the rock are still alive and thriving. And the aquarium was pretty from day one- Poof, instant aquarium
My rock survived a plane ride from Florida to Montana. And then a year and a half later, a nine hour car ride when I moved across Montana, without losing the life that is unique to the rock TBS sells.
You can buy dead rock and seed it, which will most likely reduce the bad hitchhikers. But as soon as you add your first fish or first coral, you can add unwanted creatures to your tank. Dealing with hitchhikers is just part of having a tank.