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.