Over the years I have used Pearl Izumi pro thermal arm warmers. Those are great but I always wanted something that was a touch less warm and more breathable. I found the perfect pair of arm warmers in the Castelli Pro seamless 2. They provide just enough warmth for nippy fall days, but are much more breathable than my pearl izumi ones for hard intervals. Also, I thought that it couldn't get more comfortable than the pearl izumi ones, but these are even more comfortable, and they are very well constructed with a very engineered fit. Highly recommended.
Castelli nailed it with these seamless leg, knee and arm warmers. They must use a very high loft insulating fiber to knit them. They are super soft, stretchy and very warm for their weight and thickness. Although they look small when you get them, they stretch a lot. And thus far have not lost their shape or rolled down.
Surprisingly warm and very comfortable. These are short for me (I wear 36-37 in. dress shirt sleeves) -- even shorter than other arm warmers that I wear -- but I still like them when a need something mid/light weight for 50-60 (F) temps