In reply to Alan M:
They are tiny and light, warmer than a microfleece and a lot more packable. Squash to the size of a big tennis ball so will fit in a Salomon 12 litre running sac without filling it.
I like it and when I very stupidly left my old one when away I bought a second. You do get a little (very little) down leak after a couple of years which does not fit with their other products using heavier/tighter nylons.