In reply to BONG123: "Good for -10" is very, very vague. Personally I wouldn't wear down under my goretex (and I wouldn't wear much primaloft under it either - this sort of thing
http://www.ukclimbing.com/gear/review.php?id=4447 being an exception) unless I was standing around doing nothing in the rain (in which case it wouldn't be -10).
I've just bought a very slimline uniqlo ultalight down jacket for a very reasonable 60 quid which would fit under my goretex fine, but I suspect it wouldn't work brilliant there, so why my question is why do you a down jacket under a shell?