I think the key to CGI is knowing how to frame your shot. The first Transformers film, for example, was very well done (VISUALLY!!!! Don't crucify me...) when you look at it because they didn't try to do constant full shots of all of the Transformers. You only ever really saw parts of them in close-up shots once they had transformed and the only times you ever really saw their full forms in a single frame was when they were in big action sequences so were moving so quickly that motion blur could hide the fact that they were CG. When you look at the 2nd Transformers, you'll notice a lot more full shots and long shots showing Transformers standing around and doing simple tasks and they just look fake. CG can be done well but you need a good DoP to frame things properly. Still, you'll never beat a good ol' fashioned puppet or model set for realism.