if the temps drop below 40F you need snows. your nice and sticky summer tires will be hard as a puck, you know how well pucks slide around on ice.
here's the big issue that i've dealt with, and i'm still dealing with: while having winters do give me the security of having one, i got performance winters, (mich alpin pa2s.) which does better dry driving at some expense of snow/ice traction. i got them because it really dosn't snow all that much around here, however, when it drops, it just dumps on us. (past 2-3 years anyway) but is cold all winter long, so i had to go w/ snows, just to keep my traction.
snows are only rated good for 2-3 seasons, so that's quite an investiment, but so are your high perf summers. this way, you'll extend the life of both by changing every season.
also, i have to disagree w/ eric on the A/S tires to some degree. i used to drive around in an eagle talon w/ awd, with crappy A/S "high perf" tires. it sucked in the cold. since "high perf" compunds froze up quicker than the temp needles, it had no traction in the cold. i had quite a few spinouts in that car. however, i did some reading on tirerack's user reviews, and got a descent A/S for it last winter. it was much better, but the point is: not all A/S tires are created equal. and don't just blindly put faith in it because it is A/S. also because A/S can also mean all around sucky, it's not hte best tires to keep on for your high performing m3 in the summer months. compromises all around, and that's never good on a high perf ride imo.