Building a 3D city can take time and sometimes an off the shelf solution is the best option for moving a project forward. As such the City Kit by GreyScaleGorilla is well with a look. Running in Cinema 4D the kits allows the rapid construction of cityscapes with a range of customisation options, the movie embedded below provides a good overview:
At $149 CityKit is well worth a look, head over to http://greyscalegorilla.com/citykit/ for full details.