I find the first and second image to be the best here. From a technical aspect they are all great. But I think they all lack a bit of "story" to them. The first and second to the best job of holding themselves up as a good image.
I always like to look at the 3d art as just art. Not from a technical aspect of wether or not it was a challenge to create. So keep in mind there should always be a point of interest, a feeling, or something compelling about an image that every viewer can feel something from.
Perhaps playing with camera angles might help here. Take a look at this building gallery which I like:
http://www.kleptography.com/gallery-buildings.htm There is a great deal of contrast in his colors. Some he plays with angles.
Again hope its helpful.
Will
