A friend of mine who does CGI rendering and animation in Hollywood had this to say:
"The helmet seems small and the chest too large. Not to mention the arms aren't much thicker than Downey's real arms. They should look thicker and stubbier on a real person and seem shorter. They made him quite a few inches taller. I see lots of areas with inconsistent lighting. Its too perfect to be real, thats why I think its a full CG."
I agree that the picture looks too "clean" and may be CGI, but I think that they may have the proportions right. Click on the image above and you'll see a side by side of not only his notes but a superimposed image of Downey over the armor. I'm quite sure I didn't get the scale of Downey vs the suit exactly right, but it looks to me like a pretty decent fit.
Regardless of whether the suit is CGI or real, it still looks amazing to me and is an incredible translation of the comic book armor to the big screen.
What do you think?