I have a MontBell goretex UL Sleeping Bag Cover.
I like the simple designs and light weight.
You have to notice that the goretex coating will be abraded to be thiner and thiner at some parts, but I think it will still be useful for using it under tarps or in a cave. Because of the coating problem, it will not be totally waterproof, but it's just enough.
I haven't used it in snow season, but I've never encountered any condensation problem even when vary humid or heavy raining days. I think it breathes pretty well since when folding it, the air inside comes out easily and quickly. It may because of the thin outer fabric, the air flows out quicklier than in other goretex bivy I've used before. If I use a down bag, I'll definitely bring it as well to protect the bag from getting damp.