I've used a Buff for two winters now -- temps see single digits in Jan/Feb; The Buff works VERY well, from there on up. I even use it during the hot times, because it does so well at keeping the sweat out of my eyes.
When the temps go below 50F, I'll put a microfiber balaclava over the Buff, and that works down to about 25F. After that, it's a knit cap instead pf the bala. But the Buff gets just about year-round use.
The manufacturer claims a variety of ways to wear it, and I've tried a few -- face mask, bandanna, skullcap -- and I want more of them!
Chain Reaction Cycles is the only place I've seen them, and I generally only deal with them during tax season (free shipping for larger purchases).