I have a large amount of respect for your artwork. I don't even know for sure, why. I look around a lot, like to stroll around "the very best" to get inspired.
From a technical standpoint this stuff isn't the very best, and I'm not really one to talk, but it has something BETTER than the wicked realistic lighting schemes or masterful brushstrokes. It has a style to it, and a feeling that you've been able to very strongly bring to life.
I would encourage, at the moment, two things: The first being, presentation. You've got some great work, but I feel that some simple contrast tweaking before posting it up could make it jump out and seem a lot more professional. These ten second little tweaks could make a big difference.
The second being, do more. I am without very many sketchbooks to frequent, I wish I had more of them. I want this to be one of them.