I am wearing the Smart Wool quarter zip right now. It fits perfectly, tight, but I could wear a Craft base layer under it. It's not even too long. It is so soft, I don't want to take it off! I will definitely use it for riding when I have a jacket or vest on, x country skiing as a mid layer, and alone for running or hiking.
It would have to be really cold for me to wear this as a base layer.
My only complaint is that I can't stand anything touching my neck and the turtleneck comes up a bit too high for me. I have it partially unzipped, so that's fine.