Thought that I would put my thoughts and experiences into this debate.
A few years ago I was running a 530D Sport auto. Lovely car but the white stuff was a big no no. My employer, based in France, for whom I ran a UK manufacturing unit insisted that all company car drivers in France had a set of winter, not snow, tyres. These were normally on steels depending upon the car. I persuaded them to buy me a set for the 5 series complete with I think 17 in rims. Local dealership stored them and fitted them onto the car for an annual fee of £50, for this they also changed them back and stored the other set or rims and tyres.
My wife and I were in the Lakes a month or so after they were fitted and we woke up to a good few inches of snow laying outside and still snowing. We were at the Patterdale hotel and whats just up the road from there, well of course its Kirkstone Pass. Gave it a go on largely virgin snow and we made all of the way to the top in a less than dramatic fashion. Even went down the Struggle to Ambleside for a coffee. My car was kitted out with jackets, boots, gloves, water etc so no dramas if we had not made it.
I now have an X335D Sport on standard tyres and in the snow it was rubbish, just rubbish which surprised me. Put the diff lock on and its a different animal altogether so with decent winter tyres it would probably improve even further.
Conclusion, winter tyres make good sense if you can afford to have them lying around for eight or nine months of the year but if you live in an area where snow is always going to happen and you dont have a Landrover Defender then get a cheap set of wheels and nuts with some half decent winter tyres. One caviat to this and that is the depth of the snow. Anything which is deeper than the front valance aint going to allow you through so dont bother to try unless you have a proper 4 x 4