I have one, theyre cheap and fairly light, mine has proved to be impressively breathable, I sweat like a pig and have spent many nights over the years soaking wet in an orange survival bag, expected the same but to a lesser extent in the bivi bag but no probs at all with my old down bag in it. It isnt goretex and I havent used it in the pouring rain without some additional cover but I have no reason to doubt it should work just fine, most of the higher end generic goretex equivalents work fine IME and I have other Wild Country stuff that has been very good value for money and excellent quality.
Gripes....no mossy mesh, so its either all zipped up or open to the elements, pretty claustrophobic when its zipped up and hot and I could imagine real risk of suffocation, I cant use it zipped up. Maybe condensation from breathing would be worse if you sleep zipped in to it. How does everybody else do it?
Jason