We discussed this in this
http://x3.xbimmers.com/forums/showthread.php?t=576271 thread and the statistics I saw for improved braking under 7C were insignificant IMO (like 5%). Lots of people seem to be under the illusion that as soon as they temperature drops below 7C you're suddenly so much safer but you aren't. Ice is another matter but if you hit ice you're in trouble whether you slide 30ft with winters or 50ft without. This is only based on the report in this thread so I would like to read some more stats on it. Obviously there are a lot of "independent" reports paid for by companies with vested interests
I was against getting them but came full circle and ended up getting some but this was mostly to give me the option of skiing in Europe in the future. You need significant snow and regular temps below 3C to make them worthwhile IMO.