I agree with Neil, my wife and I both have the ID Hot Sock, and they work well. I don't think they're as warm as down booties.
The downside is that, for the weight, I think at least sometimes I would rather carry a[n additional] pair of just regular thick wool socks, even if they're not quite as warm at night, because they're dual purpose ... can use during the day too, or as mittens.
OTOH, a nice thing about having dedicated warm sleeping ~socks is that you're *not* tempted to use them during the day and get them wet and/or dirty.
I've read the suggestion of making tyvek bootie shells for them to allow them to be used as camp shoes, or even just plastic bags & rubber bands, but haven't used them this way personally. I brought these with me for part of my PCT thru-hike last year but FWIW ultimately didn't use them that often just because I was usually warm enough without them, but of course for a person with a different metabolism or gear mix or hiking conditions or any combination of those, they could be a fine choice.