In reply to Mullany:
I would second what Toby said about the Full Zip buffalo salopettes, Halfzip is undoubtedly better for venting if you're in them all day.
I got full zips for 20 quid on eBay, and ended up adding extra tabs of Velcro to allow me to vent the tops of my legs without the whole sallopette unfurling.
One thing I will say, is if you run hot the full DP system may be a bit hot in all but the worst conditions. Sallopettes and smock together are substantially better insulators than the smocks alone... Ideal at 10pm in a snowstorm, not so good at noon in bright sun on a windless day.
If I was buying again I'd be tempted by techlite salopettes or dp trousers to run a bit cooler (I stay dry, not sweaty; but find myself getting uncomfortably warm on long fast walk-ins). However I instead tend to use norrona flex1 or troll microfibre windproof Kecks over thin micropile longjohns when it gives good weather
That said, bottom line, the buffalo system is the cheapest, most versatile way to keep safe and comfortable in every conceivable winter condition; I wouldn't be without mine.
Post edited at 21:15