Mavimao is right. I think Spider-Man 2 is an example of a movie that combined CG and more traditional effects very well. Most of the CG in that movie is either in Doc Ock's tentacles or in some landscapes in those instances where it would just be impracticle to use mechanical effects. But a lot of the movie is shot on locations in New York, Chicago, and Los Angeles. Real tentacles were built and used in as many scenes as possible. Almost every scene was shot either on location or on a traditional set. And the reasons...? One, real props and real locations look real, and that's important for the audiences to accept it. And that's why a lot of the shots from the original trilogy hold up. Those ships actually existed. It would have been possible to have actually touched them. They would be really small and wouldn't be able to fly through space, but those models actually existed, so they seemed real. Second, actors work better when they're actually playing against something real. Acting is hard enough as it is without being constantly surrounded by blue walls and talking or interacting with something that you can't even tell exists. Yes, it is possible and even necessary to learn to do in acting classes and auditions, but being able to play off of something in real environments can often bring an actor to a whole new level of technique and pull out performances he might not have known he could do. In contrast, almost every scene in the prequels had some sort of computer effect in it. Rather than build sets or creatures, George just kept his actors on the same blue stages over and over again just adding in background later. Why is building a set such a bad thing? It isn't. There was no reason why most of those scenes couldn't have been shot on an actual set. It's just laziness, and it shows. Third, just like Mavimao said, it's important to mix it up. Don't do every shot the exact same way. It keeps the audience guessing and hinders them from immediately figuring out how an effect was done. The constant CG and bluescreen in the prequels is obvious enough that you can't help but notice it.

In my opinion, CG is that special tool you keep locked away in a box only to be opened for emergencies. If it's not possible or plausible to do it any other way, then you bring out the computers. George, however, used CG as a crutch throughout the prequels, and it shows that he took the easy way out.