A value-oriented bivy choice for use where exposure to wet conditions is minimal: inside a tent, a snowcave, or in the desert. Conduit SL(TM)/nylon ripstop laminate upper is water-resistant, windproof and very breathable; waterproof, extremely durable, polyether urethane floor.

Both have served me well by adding significant warmth (and wind resistance) on cold nights. They are of little use in a serious downpour, maybe worse than minimal since they trick you into thinking you are prepared when you are not. In a serious rain, you would need to also be protected by a tarp.

I am pondering a more lightweight bivy (aiming for 8 ounces or so). This will cut 10 ounces from my packweight, even more if I decide to stop hauling a ground sheet around.

One candidate is the Superlight Bivy from MLD. It is 6.5 ounces in the XL size and sells for $169. It has a silnylon floor and uses a DWR treated 10D fabric for the top. Mesh in an option on this bivy.
(MLD also has a heavier (12 ounces) and more expensive ($335) product they call their "soul" bivy which is made from Cuben fiber below and Event fabric above.)

Another candidate is the Bristlecone Bivy from Katabatic Gear. It is 9 ounces in the Long Wide version and sells for $159. It includes a bug mesh panel, which is nice. It is made from Silnylon (floor) and Pertex Quantum (top). Note that Pertex Quantum is a 0.9 oz/yard nylon fabric with a good DWR treatment.

There are also bivys from Oware and TiGoat (the Raven) and Ptarmigan that I should look at.