On tread design alone, I'd not try these in the snow, as my experience with uni-directional tires is that they don't perform in snow. To me, the M+S designation means the rubber compound stays pliable and resistant to cracking in cold temps (whereas summer tires don't do well below 40 deg. Farenheit) and would stay pliable and soft enough to deal with cold, rainy weather (the worst kind of rain is cold rain, as the tire hardens). These tires are razor sharp when it comes to handling, and given the good tread wear, it's about as good as it gets for a balance of very high performance and tread life, which still be usable in cold weather (not snow).
For aggressive drivers, or ones that hit tracks now and then and can dance a car on the limit, these will suffice, but in NO WAY are they the kind of summer tire you would really want if you wanted a sole-purpose tire designed to be glued to the road and be completely surprise free at cornering limits. But then again, this tire will last, and be usable in the cold weather.