In reply to BigBrother:
I used it years ago on a down jacket. I felt that it affected the loft but only very slightly, and certainly helped in damp conditions (when th eeffect on the loft was positive).
However, you shouldn't consider it a waterproofing: it helps in damp conditions, say in a snow hole, but rain etc is still going to 'kill' your down (until you can throughly dry it out again).