For some reason, the lower left looks really fake to me... I bet it's the lighting... But the other three are freakin' awesome. You have real talent!
Odd, because the lighting levels are exactly the same in all of them. Because of the slightly reflective nature of the hull it reflects some light onto other surfaces and so areas in shadow can still have a small measure of light in them. The only parts that would be completely dark are where no light could possibly be reflected onto. The background is basically a circular wall or screen image around the model, one that I sometimes have to adjust in offset (up or down) depending on the angle or view I have of the model, but no light or reflection comes off the background. The background does have some element of depth to it because I used Photoshop to create the scene in several layers and then I finished it as an HDR image. This allows me to put the model right within a more realistic setting when rendered.