When I first started out on motorcycles I had a WWII flying jacket handed down by my dad, don't know where he got it from but he was a miner in the war, not a flyer. It was lined with sheepskin and it was really, really warm for my first couple of months on the bike (I started riding on the roads in Derbyshire in February 1969, when there was six inches of snow everywhere) but it was terrible if it got wet. It stank, and it took ages to dry. One weekend I went to a party a couple of miles from my home and when it was time to leave I found that somebody had stolen it. Never saw it again. I replaced it with a police mac from the army surplus, I think it cost ten shillings. It was absolutely waterproof and what it lacked in insulation I could compensate for with another jumper or two.
Last time I saw JonathanD of this parish he had a reindeer pelt with him. Only time I've ever touched anything reindeer. It was absolutely gorgeous, it made me want to run my fingers over it all night. Like my German shepherd cross only more so. I could tell how warm it would have kept the animal when it was wearing it.
But it must have been a small one, it wasn't anything like big enough to wrap around me, and I'm on the skinny side.
The thing that would bother me is how long it would last. A while back I bought the SO a fur coat. Going out in winter she wore it all the time. Whenever she sat in an armchair or anything like that if she was wearing it there were always quite a few hairs left on the upholstery when she got up, but it got worse and worse and after a few years it started to moult big time. That left some unsightly balding patches. I think in the end we used it for baggywrinkles. There would probably be ways to treat it to make it last better, but then you have to worry about moths and that sort of thing too.
I think a synthetic product will be more durable, it will be easier to care for, there'll be a lot more choice, and it's likely to be cheaper to replace it when the time comes.