So far, i'm of the opinion that the best bivy is one that uses primarily mesh/netting on the top, except for the knee to foot area for use under a tarp.
I know some folks rely on a bivy for cold weather backpacking to add a little extra warmth and wind protection, but i don't think the condensation is worth it. The mesh/netting will block some wind and still help one's quilt to stay in place, but it's infinitely more breathable than any full or almost full fabric biviies, which i think overall is more important/a plus.
Also, lightweight mesh/netting doesn't cost near as much as the UL fabrics that weigh around the same.
Besides, if you're into multi-use and ponchos, just get/use a Frogg Toggs regular poncho, which in a pinch can provide more serious water and wind protection for the top of you when needed and still is fairly breathable.