I should start this out with saying I'm usually the first person to tell people to stop obsessing about chemicals and to relax. (I'm a chemist...)
I won't use these things. It is a really long time to be heating plastic. I just don't like it.
If you are doing it for the clean up, we spray our stoneware crockpot with olive oil cooking spray. You can rinse the crock clean, even after having tomato based sauce in there for 8 or 9 hours. No scrubbing involved.