Important to note that there are 2 categories of non-studded winter tires, performance winter and regular. The regular non-studded winter tires are tires in the category of the Blizzak WS60/WS70 and Michelin X-Ice Xi2. Examples of performance winter tires would be Blizzak LM25/LM60, Dunlop Wintersport 3D, or Michelin Pilot Alpin PA3.
The performance winter tires trade some snow and ice traction for much better dry and wet handling/braking, and usually have much longer tread life. If you don't drive in severe conditions that often I'd lean towards getting a performance winter tire because they are a lot more pleasant (and safer) to drive on clear roads. If you have a RWD car with an open differential (like a BMW 3 series minus the M3, or a RWD Merc) you might want to consider the "snowier" snow tires anyway but it depends. Keep in mind that AWD doesn't help you stop, so don't think you can get away with all-seasons in really bad weather just because you aren't getting stuck.
I had the Michelin PA3 on a BMW M roadster and I was able to get around the NJ/NYC area just fine in any snow that the front lip could clear. I actually got rear ended in a storm by a Jeep that couldn't stop that had all-season tires! Just in case anyone crazy is wondering, you can actually fit skis up to 170cm in the passenger seat of a Z4 but they have to be put in with the top down.
Just in case anyone was considering the Michelin PA3, it's pretty much the least snowy snow tire made, they have tremendous dry grip in the cold and are quiet and stable over 100 mph. I have Dunlop Wintersport 3D's ready to be put on my Mazda 3 this year and I'll let you know what I think once they are mounted.
Oh, and if anyone is choosing between Blizzak WS60/WS70 and the Michelin Xi2, one important thing to note is that the Blizzak has a "gritty" compound through the first half of the tread on the tire, as exracer and SkiMangoJazz mentioned. This gives it slightly better ice traction but makes it wear faster than the Xi2. Once the first half of the tread is gone on the WS60 or WS70 they will not be nearly as good.
Edited by chris719 - 11/19/10 at 8:54pm