In reply to Todgerman: Got some for both cars last year. My wife found some use for them, managing to use them when she got stuck on local journeys after the "downfall" that happened heavily just before xmas (S. Wales). I was able to get out on a just replaced normal set of new tyres, to drive up to Scotland.
The temperature was -15 at night all week there, but very little snowfall (in moffat), but the roads were all compacted snow. I got around alright but I learnt to drive years ago when the weather was normally this bad and I enjoy driving on the stuff carefully.
I arrived at a farm to get the duck and turkey the year before last up a road that the farmer had said was closed for 3 days. It wasn't a motorway, but careful driving in a front wheel drive 2L 140ps diesel I found it easy enough.
Driving out the gritted track they had been using was terrifying however, in a way that I felt out of control way.
My guess is that they are great for getting home in an emergency. But I think getting used to driving on/in snow and ice is even better. We have just not got used to driving in these conditions.
On the other hand, I've bought winter tyres this year, so expect the mildest winter in years!