Last three are just awesome.
As everybody have said, you need to work on eyes especially.

Your dad is wrong with that... some art requires more than 10 hours of work, which means you need to have a break.

learn how to draw eyes, study it's anatomy, see more pics of random eyes and you should be able to make them look good

Working on a new project,this time I am using blue as my main color.But I don't know what color on the eyes should go with blue.Any suggestions?
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Finally!A new artwork with different colors than all the others.Using blue as main color.Used some Radioactive,Marine and Aqua on the eyes.C&C now!Oh yeah,I regret I sold my Metal Sadness for 27k,so sad...

i love the last one the visor is good, but the eye make it bad, btw all of your head is good bro.

Don't use gradients for thr shading and highlight, and don't do it with a sharp selection, have you ever seen a robot that has an outline around every piece of it's head that at random times changes from black to white? It completely destroys the accuracy of the highlights and shading, the highlights and shading are to show where it's reflecting light and whether it's sort of poking out of going into the head.

About to finish my next one,I have been sitting in front of my GIMP for about 4 hours.
Uploading the preview!
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Done uploading!Using new colors in this project,her name is Project Warrior.I mostly use yellow,orange and slight white on the metal plates.The eyes are kinda too small,I know it's a huge mistake(not really used to eye mapping yet).Oh yeah,C&Cs are appreciated.

The shading on this one is too heavy, think, it's meant to be like a powerful robot or something.
You know, solid, strong and stuff.
When you make the shading so heavy when it's coming from under the parts of it, it makes it looks as if it's floating.
Try to simply lower the opacity of the shading layer, and thinking about where on the parts you want it to be really tightly together with the part that's under it, or whether you want quite a bit of space between them.
Adjust the opacity of the shading in them perticular places to make it a little more realistic.
Also, the highlights are way too strong, it's as if there is a sun at every angle of the head and is shining at it, remember, one sun.
Think about how shiny the surface is, and don't just make the edges so shiny, try putting gradient through the actual pieces of the head, AlphasoniK did it on his "bass" head, now look at it in the expo. :o

Another thing is the eyes. I must say this head is much better than the others, the thing though that spoils it, is the eyes.
Think about this, it's a robot, it's not meant to have oval eyes, think about the shape of it, and make it suit the rest more.
Try making them look more intimidating but at the same time, emotionless. Show no emotion in the eyes whatsoever.

The part with many thin and long pieces sort of put together doesn't quite work, if you want to keep them, I'd seriously suggest lowering the opacity on the shading.

Wow , I wanted to comment on this but praeter just explained it

Pretty good. Especially the Alien Robot one