A few years ago when I was first getting the hang of Illustrator (after dumping FreeHand), I did a lot of vector portraits of people. I think this is the last one I did, where I really got the hang of the layers, masking, and sampling colors.
Some people might think there's not much point to vectoring like this since the software can do it automatically, though I like to think that human-drawn vectors are more graceful than the auto-generated ones. But it starts with a photo anyway and in this case it was a good photo. The real test would be to create a good vector portrait from a so-so photo, or an entirely original piece from several reference points.