One of my favorite artists - Karin Jurik, uses mug shots to practice painting faces. She has a blog dedicated to her mug shot faces called BUST-ED as well as her 100 faces project on her website here. I have to agree with her that it is a better way to learn. These people have not been photoshopped, altered, prettied, make-upped or anything. They are a face - plain and simple in its natural way with all the lines and creases and uneven color.
Want to see more of Karin Jurik's great mug shot faces? Here's a video of her work.