We have wool and cashmere duvets, which I love, and which, even sleeping with the bedroom window open, is perfect for the climate here in London in a well-insulated house.
The eiderdown is a completely different experience, particularly in a cold bedroom. It's not like an ordinary down duvet, it's literally is like sleeping under a cloud of perfection. My mum has offered us one of the two she inherited, but it would be wasted here.