FWIW, I have one of those that I occasionally pull out when it's not necessarily that cold (i.e. in the 30s or 40s), but the wind is going to be a factor. I've also worn in in races when I really didn't want the drag of a jacket, but didn't want a frozen core, either. As the previous poster said, it can make your chest a little sweaty, but the back is very breathable, and you manage to vent most of the excessive heat and moisture through the back.

The LG fully breathable LG vest is my go to base layer when temps are in the 40s and 50s, and even low 60s - adds a layer of protection from the cold, but is breathable enough to still be comfortable even as the day and my core warm up.

BB