I would much prefer to do portraits of real people rather than faces purely from my imagination. People can be funny about sitting for you though, and I guess that's reasonable seeing as how the artist must scan the person, drawing out their essence. The scrutiny and the end result is probably quite confronting. Not to mention having to hold a pose for a length of time.
I have made portraits from photographs and while they turned out to be good representations, for me the real person is where the gold is, where you get to transfer their unique vibration to the canvas.