Epic piece man. I love the way you worked the scale to have the Marine seem huge compared to the others, and the style really captures the feeling of the dark grittiness of the 41st mil. The only thing that I was not very fond of was the face, but I also understand that faces are EXTREMELY difficult and it still looks realistic and battle worn.
All in all a great piece. I wouldn't be surprised if I saw your art in a games workshop book someday.