What are people's experiences with this top in terms of layering. Is this top fine for nordic skiing at 20F? Is it only a baselayer and should have a top layer when it gets below a certain point? What are your thoughts?

I think you'd be ok in this thing for nordic in 20°F weather, as long as you keep moving. I sweat like crazy when exerting myself, even in cold weather. On a skin track, I strip down to the Bliss and some light gloves down to 10°F. If you get chilly, toss a windshirt over the top. Just be sure to layer up when you stop. EDIT: Wow. Going around answering old questions today. Sorry!

This is the quintessential layering piece. Whether you use it as a base layer, midlayer, outer layer, or the only layer depends entirely on conditions and activity. When backpacking, I wear it as an outer layer over an ultralight merino base. This arrangement is good down below freezing as long as you stay active. Warmer layers go on top once I reach camp. For higher speed activities (biking, skiing), I like to have an outer layer that blocks the wind. Check out the rime and shift softshells. With these outer layers, it is sometimes too warm to keep the midweight merino, so i wear the softshell over the ultralight merino. If it is cold enough, i will wear all three layers. They key to layering is to have many different combinations available. For me, more often than not, this shirt is in my layering equation. But to answer your question more directly, this shirt alone will probably not be warm enough by itself for 20F nordic skiing.