Condensation: My hiking partner has used a waterproof/"breathable" bivy in cold weather, and had huge problems with condensation. I've been using an old Mountain Hardwear MicroMesh bivy -- very breathable, not waterproof -- for about seven years, in all kinds of weather in the Southeastern US, from very hot humid summer nights to deep winter, and I've never had any condensation at all. So I think this depends on the type of bivy to a great extent, along with weather conditions etc.
General Usefulness: I like the bivy/tarp combo. I find it to be a most versatile combination: I can sleep in the bivy by itself, either under the stars or in an AT shelter. If the weather threatens, I can set up the tarp.