I put 19" Dunlop Wintersports on my new 650.
So far, I've been impressed. I've used the Dunlops on a couple previous E65s and always had great luck, as well.
I'm in Columbus, Ohio, so we get some snow, but not ungodly amounts.
'08 BMW 650i Sport -- TitaniumSilver/Black
'07 BMW X5 4.8i Sport -- SpaceGray/Black
'06 BMW 325xiT -- SparklingGraphite/Beige