I have a pair of these. 240 spike option
http://www.conti-tyres.co.uk/conticycle/ti SpikeClaw.shtml
When its going to be icy I swap them over and then leave them on until the weather changes back to normal.
They are the nuts and brilliant in snow. Hard work on tarmac but I have yet to come off.
I remember going home one evening and after about 500 yards thought there was a lot of shiny standing water... but it was all ice, I never noticed until I was in the middle of it and just kept going.
They are very good in snow too.
The only problem is every winter I keep meaning to get a spare set of wheels and have them ready but I am such a tight arse Yorkshire man I have never got round to that, but they are only on for about 2 weeks a year and are hung in the garage still looking like new.