I've got some Under Armor microfleece that is my warmest base layer, so I only wear it on the coldest days -- eg, when my morning commute is about 25 F and colder. On most winter days I wear Craft base layer, their mid-level warmth version. I've got some cheapo wool blend base layer that I got at Target that is actually very warm and great value at $20 for long-sleeve shirt. It's comfortable in temps ranging from mid to low 20s to mid-50s -- about the same as my Craft base gear.