I used a down bag for years as a mountain climber. It was always great, very warm, and certainly did "scrunch down" into a compact volume. I was always super careful to keep that bag dry. Double wrap in plastic trash bags!!

good sleep is VERY important in the wilderness. and you won't get that if you are tossing and turning because you are cold. buy a warm bag!! can be down or synthetic .. but go warm!!

I think there may be two different things being discussed; first, bags made with waterproof/resistant shell fabric, and second, bags with the down filling itself treated to be water resistant.

See, e.g. http://www.mountaingear.com/themountainblog/2013/06/the-scoop-on-waterproof-down/

Water resistant down-

"We went the second week of sheep season, which as it turned out was a less that optimum choice, and we caught 6 straight days of rain, cloud, and fog. That made for some pretty lousy hunting, but it did however, make the perfect field test for finding the limits of waterproof down.

Day 1 and 2 consisted of intermittent drizzle and fog. The temp hovered between cold and dang cold, so the jacket got used a lot.



As advertised, the down held up to the sweat and 100% humidity amazingly well. I was impressed.

Day 3 it settled down into a steady cold rain that didn't quit. If you've ever hunted in the rain for days in a row, you know that EVERYTHING eventually gets wet. Water runs down your sleeves when you glass, gets down your neck, soaks the hem of your jacket when you bend over, and of course you get to sweat in raingear all day. Despite babying it to the best of my abilities, this kind of non-stop sogginess was the death of even the waterproof down, and by the end of the day sleeves and fringes of the jacket were soaked and useless. Although the body still had some loft, the jacket had lost a large percentage of it's insulation capabilities. If I had been relying on this as a critical layer, I would have been screwed at this point."

http://www.rokslide.com/forums/showthread.php?10158-quot-Water-Resistant-quot-Down-tested-to-failure

That is indeed useful information, but in all fairness, finding that a garment will eventually be wetted through while in use in constant rain is not representative of the way in which a down sleeping bag is likely to be used.

I did have an interesting night during which I slept out in a down bag (standard untreated down and nylon) inside a bivvy bag during a night of alternating rain, fog, and snow. I sensed that the loft of the bag was decreasing, although dawn arrived before it got to be really bad. My take on the situation was that I would have been warmer if the temperatures were cold enough that I only would have experienced snow.

In cold, rainy conditions I really like synthetic fleece for outerwear, especially underneath a shell garment. I can then anticipate a warm night in my dry bag, waiting for me back in camp.

Or wool. Heavier, but warm when wet and durable. I am a fan of wool sweaters, socks, and long underwear in the outdoors. I love my wool overcoat when in an urban setting. My overcoat stood up to subzero temperatures for several hours a few years ago, walking around a city. My feet suffered, but the rest of me was tolerably warm.