I believe they are really good, my friend is from Germany, and they use them every Winter there, they have much more snow than we do, and he says they are a must for them!......their Insurance demand that they use them!......
if you get months of snow it's worth having a set on another set of wheels and changing them over each winter but for most of the UK I'd say not worth it. They are better, definately but just be careful on he normal tyres.
They should only be used if the temperature is below freezing, as soon as the temperature goes a few degrees over freezing they wear out very fast . This is due to them being made of soft rubber that gives a good grip in the cold icy weather. This rubber is so soft it wears out with in a very short time if used in temperatures above freezing. It also gives poor grip in normal conditions. In places like Germany / Sweden etc where the temp stays below zero for weeks or months at a time people have 2 sets of tyres and wheels. There are announcements on TV and in the press about when to change to winter tyres and back again. Definitely not recommended for the UK where last years winter was an exception , when it could have been a good idea for a few weeks.
This was discussed a while back and even a senior technical director of Bridgestone tyres said they were not suitable for the UK unless conditions were exeptionally bad for several weeks.
Best bet is get good quality normal tyres and make sure there is plenty of tread on them when the cold icy weather arrives (if it ever does)