I've tested the AMK bivy units a couple times in spring/fall conditions, and have been pretty comfortable. The only problem I've really had has been with condensation, which is why I think it's not just the bivy, but, making sure you have appropriate layering on anytime you're out there.

Here's one of my campsites using the bivy, and poncho, and a contractor bag as a ground cloth under the AMK bivy: