I use a mixture of vermiculite and perlite. If you want to use it, mix one cup of perlite for every two cups of vermiculite. The perlite is supposed to help with the airflow around the eggs. I'm not sure if it does or not, but it's what I've always used and it has worked great for me.
On the Hatchrite, though, I too agree with Bob. I've heard some people complaining that it isn't always perfect. Use your vermiculite or vermiculite/perlite mixture and oisen yourself. Add water to it, but squeeze it in your hand--if it clumps together but breaks apart easily, you're all set. No water should drip out of it.
