I have Sugoi's MidZero Zap tights (running tights), which work quite well in the 40s and down to the upper 30s, depending on the wind. I've worn them in the upper 50s/ low 60s too (it wasn't supposed to warm up that much!), and they were almost too much, but not horribly uncomfortable. (Of course, I'd also taken my gloves off and unzipped my jacket...)