So far I only had to leave the bag in the car for just a couple hours, but the racquets were in the thermoguard compartment with the covers on, and I placed a frozen bottle of water (that was put in a ziplock, for added protection), and was placed in the thermoguard side with the racquets, so I'm sure that the racquets kept their cool. All seemed fine and my racquets have all gut set up.
But I just had to send my wilson bag to wilson, as the inside fabric on the moistureguard side tore on the inside seam, and I never even used that side as yet. I do hope that they make good for it as it was less than 2 months old.