Wow, pretty solid. This should be a pretty nice contender for robot day.
I like the consistency of the color. You have kept up the sepia tone very nicely.
Your lighting is very good. Normally I would make mention for you to make much stronger and clearer highlights, but then that would have been inconsistent with the style if the piece. You have done well with giving metallic looks to certain areas, which is very nice on its own.
Over all I like this piece, and its mostly pretty well done, except for one part, and that is the duplicating of the robot. Sadly its kind of obvious that you have the same bot copied and pasted for quite a while. I don't blame you, it would have been a lot of work to paint a different robot each time. I would have been looking for a short cut as well. What you should have done, and can still do very quickly and easily (especially if this is done in layers) is use some gradient overlay to dull out some of the color of the robots in the back. You can also take a few minuets and try and add a couple of marks on different robots to try and separate them. I understand that they are fresh out of the factory, which I can see in the background, but you gotta separate them a little, or you will get this repetitive look that will stand out really bad.
Very nice. Good luck on Robot Day!