In reply to Ricky Martin: I use PHD Synthetic Combi bag but it is one from the sales 18 months ago, it's not quite the same as the one they sell on the site now. Drishell cover and a very room cut. I use it with a Minim 300 which has a comfort rating of 0c on its own and if I remember rightly the Combi was supposed to add another 5c. Can't remember exactly but mine weigh around 1400g together but the 300 has a full length zip so I can use it as a quilt in summer - this adds 110g.
I slept in them at -10 quite happily in just pants and socks last winter, depends on how you sleep. It's certainly a more flexible system, the doris got cold in the night and she ended up with the Combi too, I ended up in my big Haglofs jacket and the 300. This led to a peaceful nigh in the end... she'd have moaned rotten otherwise. You can't do that with a normal bag!
This summer I used the Combi on it's own for a bivvy and again for a camp where I knew there was a good chance the bag would get soaked. On it's own it's not the warmest thing but it is a bit too roomy I guess.