I've been keeping clear of these debates until I got chance to try my Avon Ice Tourings in proper snow and ice, and I have to say I'm mighty impressed. Gives more confidence when roads are covered, and much better at setting off, corners and stopping - tried it against the 'all season' e60 which was on/off the traction control and ABS like mad, whilst the zed relied on traction from the tyres.
Not too scientific I know, but we now use the Zed in the snow instead of the e60 if we have a choice.
Got 225/40 R18's all round for £400 delivered a month or so ago.
p.s. Not so good at a 9 degree run up the M1 a few weeks ago though - slippery on 'normal' roads after a couple hundred miles at that temp!