Depends on the top fabric and any ventilation options. My hiking partner bought an Epic bivy from a cottage gear maker, and had *huge* condensation issues. He was basically soaked inside after one night. On that same night, sleeping only 20 feet away, I was totally dry inside my homemade bivy.

His bivy was made using the "standard" bivy pattern, which was designed for a waterproof mountaineering bivy. It has a slit across the chest for entry/exit, and a 4-inch wide strip of mesh across the eyes and nose. Once you are inside, there is very little or no ventilation, so all your exhalations are captured inside and cause heavy condensation. I also suspect that the Epic fabric is not very breathable. (He also sweats like I have never seen <grin>.)

My homemade bivy uses Momentum .90 fabric from thru-hiker.com. It has a huge mesh panel over the upper chest and head. There is plenty of ventilation, and yet plenty of protection, too, when used under a small tarp. I have a zipper that wraps around the outside edge from my right shoulder to my left hip, for ease of setup and entry/exit. The bottom is silicone nylon, for durable waterproofness.

I haven't tried all the commercial bivies, so I can't really comment, except to say: look for good ventilation. You don't need a full coverage, waterproof bivy (which you already know). Just something to add some weather resistance, warmth (in winter), and bug protection (in summer).