This is looking great. For such a soft value range I think you have captured the values very well. I think some more painterly brushes would go a long way in adding interest to the background and surface of the piece. The edge work on the piece is very nice. I think right now the hair could use more shape carving so it has less digital airbrush vibe going on and more observed shape definition. That's all I can see at the moment.
Keep up the great work.