What do you do with this ... thing?
Well, it was originally designed for use in third-world countries where fuel for cooking isn't readily available. You can heat a pot to boiling, put it in the Wonderbag, seal it up, and let it slow-cook for hours.
How long it retains heat depends a little bit on how warm your environment is. If you live in an igloo, it's going to cool off a lot faster than if you live in a normal house.
You can also use it to keep cool things cool.
I reviewed it here, if you want more of my thoughts about it.
Wonderbag has a new holiday version that you can find on Amazon.