These are a good compromise between thermal arm warmers and lightweight warmers. Have ridden in 40-50 degree temps while soaking wet and they kept me warm. Also ridden in 60 deg temps and not uncomfortable. Ordered size medium and 157 lbs., 5-9, fit is snug but they don't fall down.
I have several other armwarmers so I'll compare this one to them. I have a few Pearl Izumi in size Medium, but I ordered size Large in Castelli and that was the right choice. It fits pretty snug but not uncomfortable for a few hours. I'm really not sure if it's warmer or not, but I think it looks a little better. It's a keeper :)
Got this simply because I didn't want to wear cycling winter jacket and over heat, come home drenched in sweat. Days that dips passed 30 degrees I wear these and thermal L/S base layer fleeced thermal jersey and wind vest and I'm good.