In reply to Dangerous Dave:
Where I'm living at the moment you need winter tyres on from the onset of the winter weather, if you don't have them and are involved in an accident your insurance won't pay up. Likewise, if you end up blocking the road and the police come along they will fine you for not having them on.
In October I had a full set fitted to the 4 wheel drive that I own. The snow at the moment is quite deep and not all of the roads are cleared, but some of the roads have a slush during the day, and ice in the evenings and through the night; temps haven't been above zero for about two weeks now, tonight forecast down to -14C.
Having painted that picture of the conditions I have been using them in, I can tell you that I am extremely impressed with Winter Tyres, and I never thought I'd say that. I have driven easily and without problems in 2 wheel drive, where I know I would have had to select 4 wheel drive previously. I am driving 60kms to the airport tomorrow morning at 4pm, and without winter tyres I might have been a bit concerned about the journey, I have them fitted so I'm not really bothered about the journey.
However, they are not a licence to think that you can up the speed and be impervious to losing grip. I have noticed a lot of the locals seem to think they can drive at normal speed in bad conditions, and that they will be alright. Not the case.
The downside can be storing the set you take off. Especially if you have left them on the rims, as they are quite heavy.