Lots on the net...Any of these products will insulate well. The only thing that makes Thermix (or any of its competitors special) is the addition of some Portland Cement, which you could do yourself (anywhere from 6:1 to 10:1 BY VOLUME, I believe). You wet it slightly (not like concrete) and pour it down just like the rest.
The advantage of this is it holds the liner centered in the chimney, giving it a little rigidity. It also keeps most of the water out - vermiculite and perlite straight can absorb a lot of rain if there's a leak in the top cap somewhere. Neither of these things is harmful, but they do reduce the insulating value.
If you want to take it out you still can. It's pretty weak - you can shake it about to break it up and then vacuum it out just like raw Perlite/Vermiculite.
It's hard work moving the stuff around. I tried all sorts of tricks. Best thing for me was a plastic recycling bin with some tape over the bottom holes. YOu can mix and pour right from there, and the size is about right for the weight I'd want to be carrying around a roof.
Mix your own is cheaper...
Portland keeps it more solid...
Like making footpaths with a tiller and portland cement mixed in...