I've been using a
Devon Duvets wool-filled duvet [also available from John Lewis, iirc] for about 6 months now, and I really wouldn't want to go back to anything else.
Wool is apparently better at regulating heat than other duvet fillings, which means that you don't get those too-warm-must-kick-off-the-duvet moments. I've certainly found this to be true; the duvet always seems to keep me at a very comfortable "cosy but not toasty" temperature in both summer and winter [particularly impressive given that I bought the non-winter weight].
And because less wool is needed to fill, it drapes better and doesn't weigh you down; also it doesn't clump. And of course it doesn't shed feathers, so no spiky surprises and less hoovering.
The only downsides [no pun intended!]: it's a bit more expensive, and for the first week or so, your bedroom will smell of lanolin. But I was brought up in Wales, so that just reminded me of home
