Well, I've a mixed goose and duck down one which I've had for thirty-seven years and which I'm still using, and it's fine. It's my winter one, 12 tog. At the moment I'm using a 9-tog one which will probably see me through the summer except for a few warm nights which may occur sometime. Whopping 15°C max here today and below 10° at night. On very cold winter nights, I wear a cashmere jumper over my pjs, and socks, and a hat (well, it's a snoody thing, actually). Snug as the proverbial bug
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So, in short, down duvets are good value, I think, though an expensive outlay initially.