I ride a lot in the winter here in Utah as long as the roads are clear. Ive been out in 20 degree weather and these wool socks from Castillo keep my feet nice and warm. They ate great under shoe covers and standard cycling shoes.
Fit is true to size. I'm normally a size 12, or 46/47 depending on sock/shoe sizing. The weight-to-warmth ratio of these socks is awesome. That said, I have not found a wool sock yet that is perfectly warm, but my feet get cold quite easily in 30-40s temps. I find myself wearing 1-2 pairs wool socks, winter boots (Fizik X5) with wind block toe covers and it takes a while for my feet to warm up. No sock is really enough, but the Castelli socks are nice in that they do not add a ton of bulk. The price is also good when compared to Swiftwick and other brands. I do wish the cuff was a little longer, like other brands.
I've worn this sock out cycling without booties down to 30 degrees and it keeps my feet warm. It's thin enough that it doesn't crunch your toes in your shoes. I've purchased three pair, and am not happy that they're apparently discontinuing them.
I got a bunch of these and the Venti socks on sale, and loved them but sadly noticed that the bottoms are not at all durable and become threadbare. I hope Castelli learns how to reinforce a sock like everyone else. I was extremely disappointed by these.
Like the title says, this is the best merino wool sock that I own. I bought a very similar pair from the same brand a couple years ago and those are still holding up. The cold wintry months demanded I buy another pair, and I did so with no hesitation.