While you followed the source image somewhat, I can see you deviated from it quite a bit. I'd maybe pick a smaller chunk of the image.
I think you got some good use out of the white gradients used for the sheen of the metal, however I think at some points I think it's a little overdone- like with the red coming up from the bottom. You've got the upper half of the form down, I'd love how the weapons appear as well.
It'd be nice if this was a bit wider/larger, closer to the 500px wide, so we could see more of the details of the piece. I like how you played with the metallic look on many of the pieces, my only critique would be that you lost a bunch of your light source in making it more of a generalized shine. In the original, the light seems to be coming from the left, especially noticeable on the helmet and left shoulder guard. All of the piece does feel nicely blended, except for the shine on the chest plate, which is very stark in comparison. Perhaps either tone it down a smidge, or add other values to soften that a bit? I like that you chose to add color in as well.
I think you lost some of the pencil texture when you color most of the shield black. If you go back in and make a few gray tints in your armor, I think it will help your picture look less flat. Also, I would like to see a closer image as well.
Very nice detail to the metal (nice work with the gradient). I like the reflection effect as well makes it look very clean cut. The only thing i would do is crop of the bottom a little more feels like there is to much room on the bottom.
You really picked a detailed image to do! Good job closing in on the face as it's got a lot of great details to work with. I see you found the gradient too! Getting that effect is nice, and it would be cool to see how you could get some additional subtle effects with transparency, too! I think it would be cool to see this one without the linework, too! Since you have such geometric shapes in this guy's armor, you might be able to create pieces that just have color-against color - and work with planes and shines. This does look nice and clean and it looks like you've got a good handle on the pen tool - great job! I would love to see how you would add that hot-white side of his face back in. it's neat to see where you deviated from the original, It would have been cool to see how you worked with the original lighting more! Still, I like what you're doing and this'll be a great place to keep going as we get more into Illustrator! Also - why are these guys so small! Let me know if you have any trouble loading it to flickr- I can help you get this to real size so everyone can see your work better. Can you switch that up for me when you get a sec?