Here's some advice from somebody that isn't in the industry. Take it for what it's worth. I'm no expert but I've looked at a hell of a lot of portfolios so here goes.
Your work is great. I especially like the creature designs. (That is the stuff that I always look for.) I don't think you should go back to school. Build on your strengths and improve your weaknesses through practice. I've heard it said that you're only as strong as the weakest piece in your portfolio. The idea is to get everything up to the calibur of your creature design.
I think the weakest part of your portfolio is the figure drawing. If you want to draw concepts for a living that will have to be improved. Their are plenty of figure books from the old masters to start with. Sneak into some figure classes at a local college. You'll have to forgive my honesty, but you'll never improve if I just stroke your ego. Believe me, I'm no Andrew Jones myself. Like I said before, I haven't even broken into the industry yet.
If you are more interested in the 3D modeling part of things then from what I've seen you don't have to focus as much on the figure drawing. (It's still important though and it almost always gets looked at.)
Those are some great comments Shmendrick, I really appreciate it. I know I nead to work on my figure drawing some more... I'm actually suprised that you said it's my weakest point, I've always got a lot of credit for my life drawing. I'm going to be going regularly again, starting this week... so hopefully I'll be better. I'll make sure to post some of them, so you can and other people can give me pointers.
Could you tell me what creature designs you like the best? It'd be really helpfull. Thanks a lot man.
I'm glad you got my reply. Don't let those bums at school stroke your ego too much. Your figure work may have been the best in your class but now you've gotta compete with every illustrator that's ever graduated and looking for a job. Take a look at some of the sketch book pages of people that post here. You'll see what you're up against. Take a look at some of Android's figure work. I went to school with the guy and let me tell you....when we graduated his portfolio reflected some of these guys that have worked in the industry for years.
I particularly liked the Gntmonster.jpg and the Kiro.jpg on your website. They're nice and loose. I think the more refined cleaned up work looses it's life and flattens out a little bit. Keep those sketchy lines in. These guys like to see how you arrived at the final sketch not just the cleaned up version.