My experience is that all of these waterproof/breathable fabrics, including Goretex, are waterproof if properly seam-sealed and Goretex is particularly durable (no delamination as with urethane coatings). Under optimal conditions, all W/B fabrics breathe adequately. Optimal conditions means above freezing temperatures with a tarp for rain-protection, so that there is a temperature gradient between the inside and outside of the bivy to push any moisture out. My experience using bivies is with Outdoor Research models, made of Goretex, which I slept in for about 200 nights total during two hiking seasons several years ago. If you plan to use a bivy without a tarp, then you will get heavy condensation inside when it is raining heavily, because nothing breathes when it is covered with a film of water. If you plan to use a bivy without a tarp in the rain, or in sub-freezing temperatures (like in a snow cave), then you might want to consider using a Polarguard bag/quilt as well.
The big problem with using a waterproof-bivy is that it will be a sweatbox in summer. This has nothing to do with being a hot or cold sleeper. Almost everyone will feel uncomfortable trying to sleep in a waterproof bivy at 80°F or above. I no longer use waterproof bivies because of this problem. Rather, I use a bug-bivy with a tarp.