I don't have a whole lot of nights in my hammock, but everyone of them were with a hammock sock at 40 degrees or below. I'm totally sold on using one, and I think they work awesome as part of a complete system. A couple of nights were colder than I felt comfortable using the hammock with the insulation I was carrying. One night I just slept under my quilts, while the next night everything went inside the hammock sock like using a bivy. That made a world of difference warmth-wise.
If I did another thru-hike, I could see using making 2 socks. One would be nothing but .75 nylon for cold weather use, and the other would be .75 oz nylon with a netting top for the bug season.