I currently own a B7 S4 (V8) Audi which has quattro 4wd and I have my winter tyres on. While I live in Wiltshire, there aint any snow or anything, but they really did make a difference in the cold weather for traction. They really are a very very good thing to put on a car - especially one thats rwd! They can actually be used quite well all year long in the UK, but they do have slightly less braking ability than summer tyres in the hot dry days, but its not far off. In the winter/cold mornings, there is no contest.
i have also found that mine did have more road noise (especially on tight, low speed turns) than my summer tyres. I cant remember the make of winters that I am running. My summers are usually goodyear eagle f1 asymmetrical 2.
Also, dont bother buying them brand new. You can find second hand tyres with minimal usage for a good discount. My winters were second hand from Germany (got it on ebay uk!) with nearly full tread depth on all four. They still had the tyre hairs! And I saved about £400!