It's not the method used to create the art that matters; it's the skill and style of the artist.
So far the d20 industry hasn't used any of the better 3D artists.
It has used some good digital artists on things like the covers to the core rules, the stat books, and so on.
The only 3D artist I'm aware of in the d20 world so far was in a few Mongoose books.
The works the artist used in those books were way below the quality he normally posts online (I tracked down his gallery and there was a vivid difference) and his sty
le just didn't look good for the book.
He used older, cartoony models and rendered them in a 'furred' like way...
Once again...
It's not the method of the art that counts, is the style of the artist.
I urge anyone here with reservations on 3D art to spend a day browsing through
http://www.renderosity.com
You will see a vast variety in skill and styles.
Some of them look very 'plastic' and would not do well in a book. Other's are amazingly beautiful works of art that can grab the fantasy feel as well if not better than most 'line' artists.
3D programs are NOT 'click to be an artist' despite what some may think. It takes skill and talent. Just like buying a set of paints will not make you an instant Van Gough or getting a camera will not make you an instant Ansel Adams...
If you have reservations about 3D or CG art; it's only because you haven't been looking at the right artists.
As some of you know I am a 3D artist, you can find my gallery in the sig below, but much of it is 'not for viewing at the work computer'...
If you compare my older images up there with my newer ones you'll see a big change. I don't consider myself one of the better artists at Renderosity though. I'm still an amateur. Hopefully someday the stuff I do will be good enough to make me proud...
As for that 'too clean' look... see some of my 'sketches'.
My second post in this thread:
http://www.enworld.org/messageboards/showthread.php?s=&threadid=10031
links to some 'sketch' style work that is not what most people associate with CG or 3D work. Mature warning on those images though.