Does anyone use one of these? I saw their ad in Muscle Car Review and it seems like a great idea. My garage is unheated and I was thinking of finding a heated storage unit to rent this winter but this may do the trick.

http://www.carbag.com

They sould like a good idea but I wonder if fumes would build up inside from our vented gas tanks.

I don't, but have you seen the other one that is an "air chamber"?
http://www.airchamber.us/index.htm
There was another one I saw like this one that inflated. Here it is.
http://www.carcapsule.com/

I like the carcapsule idea. Pricey though!

From what it sounds like, both of them circulate the air, one is already formed and the other inflates.

I have a car bag that the previous owner used for my car. You drive onto it then zip the car inside with a can of dessicant to soak up any moisture inside.

It's been in my attic for 8 years. I work on the car in the winter time and drive it in the summer, so I've never had a chance to try out the car bag.