Hot and steamy!
I have a lightweight Capestorm bivy which is almost always with me. It is breathable, but it is very difficult to totally prevent moisture build-up inside the bag - even with the very good, Gortex ones.
Friends of mine with the identical model have endured a substantial storm in theirs and reckon it's fine (except for the condensation). I would avoid sleeping right in the open, even with a bivy bag - some kind of protection can always be found somewhere. And it is a good idea to test the bag with a garden hose before the time.
I know someone on this site that has a Gortex Mountain Hardware bag...let me see if I can get him to write something about it here.