In reply to ADJC: I've had a 3/4 lenght ultralite thermarest for the last 17 years. I use it for all 3 season stuff, including many nights sleeping on glaciers in the Alps, then for winter use I add a ridge rest, but any similar cheaper copy would do the job.
Unless you are ridiculously tall don't bother with a full length, just put you rucksack under your feet.
Having two mats in winter, one foam, means that even if the thermarest punctures you'll still have some insulation, plus it protects the thermarest, plus you can use the foam mat outside a tent for sitting on without worries.
I've had only two punctures in thermarest in 17 years - if I add all the trips up I've maybe slept something 1.5 years of that 17 on that mat! - but we bought cheap, thick, full length thermarest-copies a year ago for car camping and one of them is already leaking. Maybe luck - i'm not sure - but it may be you get what you pay for?
I should add I live in Finland, and camp and bivvy in winter each year - normally in temps between -10 and -20. So believe me, I've had plenty of time to think about any weaknesses in my system and regret them! Some pictorial evidence of the fun of winter camping in these posts (scroll down):
http://lightfromthenorth.blogspot.com/2008/03/home-from-norway.html
http://lightfromthenorth.blogspot.com/2008/01/weekend-climbing-photo-essay....
http://lightfromthenorth.blogspot.com/2007/01/ice-climbing-at-helvetinjrvi....