Dimensions is great for certain things.  It is one of the few programs that
creates Illustrator bezier curves from 3D models. Because it can render the
outlines of an object as bezier curves, I found it very useful for creating
black and white line art, in Illustrator format, of relatively simple objects.
If you want  vector output from 3D models for use with Illustrator (or similar
programs), Dimensions is a great tool. For general 3D modeling, Erik is right,
there are much more powerful programs out there that can do more realistic
rendering than Dimensions can.
-David