Overall, I enjoy this sketch. If I were to critique it with realism in mind, I'd tell you that the proportions are a bit off (which is one of the most difficult things to get right). However, I'm going to look at it with no particular style in mind and say that it's a bit quirky, fun, yet real; the breasts sort of have a 'Picasso-esque" feel about them. That entire area of the piece really draws me in and I think you could definitely turn this into something bigger (focusing on this point or mirroring another focal point above the head). Nice job on the shadows/highlights as well. I'd like to see more with this!
"Those who dream by day are cognizant of many things which escape those who dream only by night." - Edgar Allan Poe