If you happen to find a real eiderdown in your travels, and you live in a cold climate, snap it up immediately. You will never go back to feather/down comforters.
On these chilly nights and mornings, I am so glad to be able to snuggle under my eiderdown. If you don't know, eiders are actually a northern sea duck with especially fluffy light down feathers, and the origin of the word is from the Norse languages.
I have two heirloom eiderdowns which are still as light and warm as you can imagine. Eider down is very lofty and light and can last for decades if properly taken care of. You can wash them in a machine, but add tennis balls to re-fluff the down.
When I was in London on some trip, I remember going into Alfie's Antique Market and watching some posh couple trying to bargain with the dealer for a matching set of eiderdowns.
Many of the vintage eiderdowns have tightly woven cotten covers, often in a paisley or flower print or sometimes a shiny satin (which slides right off the bed). You will sometimes see quilts advertised as eiderdowns, but unless they're filled with eider down, they're not.