I have made a bivi out of a driducks poncho (tan color). It seemed as a good material, the raingear made out of this material is said to be very breathable. It's also cheap, so why not to try.
However, the bivi failed badly in the field. I had quite a lot of condensation on the inside of the bivi every time I used it. I never stayed in it for the whole night, just to the point I realized that I will do better without it. The conditions were quite good for bivi use (no rain, just strong cold wind once, around freezing the other times). A friend of mine using a bivy with gelanots membrane stayed completely dry in the same conditions. I was quite cold on one of the nights, so it is not due to excess sweat. The result was probably the same as sleeping in non-breathable bag (as far as I remember it).
Is there someone with similar experience or someone for whom it works? I'm just curious, my next bivy will be made of something much better. At least I have tested the pattern on a cheap prototype.