Rating: 3 / 5
I have a few Epic bivies, one from Oware, one from BD (Lightsabre) and one from Bibler (this one, the Winter Bivy). They are both great in the summer, especially in rainy weather, where the Epic treatment remains durable and water rolls off quite well, never entering through the fabric.
And then you have "normal mountain weather".
Cold temperatures, below but still "near" freezing. Let's call this range "15 F to 25 F". The condensation in an Epic bivy at this temperature range is miserable. While Epic is a superb fabric for keeping stuff out, it's not so breathable when it counts. Compared to something more waterproof, such as eVENT, or even XCR, at these temperatures, and Epic can't let enough moisture out to keep you even reasonably dry.
At temperatures colder than about 15, things get better again, because the condensation simply freezes on the inside of the bivy and it brushes away rather well in the morning.
So, if the bivy is to be called a "winter" bivy, it's ironic that a typical winter condition (15-25F) becomes its Achilles heel.
My larger gripe with the bivy is not the fabric but the sizing. Again, fine for summer: thin bags, little clothing. But add a fat winter bag and a down parka and pants, and there simply isn't enough room to keep everything nice and lofted for the typical near-zero winter night. And I'm not a big guy (5'8" 160 lb)...
I'd be more inclined to rate the bivy higher if it was renamed the Bibler Summer Bivy.