I've limited experience with windstopper fabric. It is not Goretex fabric and does not have the same properties but then again there are more than a dozen varieties of Goretex so it's like comparing apples to grapefruit/grapes/lemons/pineapples instead of "just" apples to oranges.

It is used in the foot and hood of my newest down sleeping bag. I've also seen it as the bottom layer in some sleeping bags. In my experience I seem to get some condensation on the foot of the bag, either from touching the tent walls or directly from condensation. It isn't a lot of moisture but it is perceptible. Since going to the windstopper fabric, the foot of the bag is definitely dryer but is that the surface or the actual down contained within that is dryer? I don't have enough definitive evidence. The bags have different ratings and quality of down. And compared to what other exterior fabric? Surely there are other solutions that protect the down from condensation but my only practical experience is from one bag. It suits me well enough.

With windstopper fleece, I have more experience as I have had a fleece toque (NorthFace) and set of gloves. Both are far superior to ordinary fleece or wool for warmth and blockage of wind. They are thinner and less bulky than other clothing for the equivalent warmth. Waterproof? Hardly, but somewhat water resistant, nothing at all like a true water-repellent fabric. Breathable - yes.

I lost the toque a few years ago and tried to find a replacement but NorthFace doesn't make the same style. Essentially the beanie portion was layer of windstopper sandwiched with ordinary fleece and the bottom 2" was just ordinary fleece. This meant the top of head was quite warm but I could also extend/unroll the basic fleece down so that it covered more of the ears and eyes! Why was that valuable? I used it for sleeping at night, the collar folded down, it was quieter and very comfy while it also shut out any stray light if I hit the sack before complete darkness or if the sun rose too early. Blocking out the "environment" led to longer and more consistent sleeps. I haven't found quite the same replacement yet. I've found another toque (lined wool) that matched it in warmth but at the cost of more weight and bulk.

I have another windstopper fleece toque from Outdoor Research which is much thinner than the NorthFace one but because it is only a single layer and not sandwiched with ordinary fleece it is not as warm or as comfortable but is more compact.