This is a really good piece. I kind of want to see a color version of this. I love your style. I like the different textures you used. This is a very well detailed piece as well as very action oriented in its own way. I like how you filled every square inch of the canvas. The legs seem a little long, but I get the sense that any flaws I'm seeing in here are actually intentional.
If you are looking to improve I would try a more complete shading to display depth. Play will some stippling or cross-hatching and see what you come up with!