In reply to Ardernhb: Ive got an Alpkit Alpine Dream (700 I think, not got it here to check). Ive slept comfortably with no clothes down to -9C on The Ben. For the price its really superb. Its been used now for almost 4 years (a couple of 3-week trips, 5 or 6 1-week trips and lots of weekends) and still going strong.
It is not at all water resistant in any way. I have used it to bivvy in the alps and the UK 5-10 times, with a Rab bivvy bag (the cheap (£30-40) pertex one that is now made from something else, name escapes me). In a couple of light showers it has been fine (ie the bivvy bag kept the water out), and at cold temperatures at over 3000m with a high wind it has also been fine (seemingly warmer than my mates who had the same bivvy bags but other down bags, not sure what type). However, on one memorable occasion at 3300m on the Weismeis we got stormed on all night (rain, snow, sleet), and it was a total disaster - we were totally soaked and freezing (limitations of pertex bivvy bag).
Having said that, since the water all got through the pertex and then pooled inside the bivvy bag, I really doubt any water resistant sleeping bag could have kept it out anyway - you would have needed a sheet of plastic. I think unless youre not using any bivvy bag at all, having a water resistant sleeping bag is probably a bit pointless. The only benefit I can see would be you wouldnt need a bivvy bag with it in a light shower...but then you might stuggle to properly cover your face?
As an end-note the Alpkit one did dry out ok in the end by the way, although there is one small patch of it which I think has never been quite the same.