If it's not possible to do it 100% practically, you should aim for part practical and part CGI. All-CGI is a last resort.

For example, the space battle in ROTJ would look fantastic if it had been shot the exact same way, but the compositing had been done with modern computers. The space battle in ROTS looks like a video game.

For a non-Star Wars example: Instead of Jar Jar-ing it, Guillermo del Toro used a man in a suit with CG alterations for Pan's Labyrinth, and that looks 100% seamless.

Look, physical models will always look more real than CG, because they *are* real. They're physically tangible. CG is, by its very nature, not. CG should be used to enhance things, not to create them from sratch. The exception is if what you want to do is completely and utterly impossible any other way.

I know someone brought up I Am Legend as a negative example, but look at the opening scenes of a devastated New York City. Except for the CG animals roaming around, which look awful, showing New York City like that would have been impossible without the use of CGI - and it looks pretty damn good to me. That's something that was necessary for the story that couldn't have been done any other way. The "vampires," on the other hand, look like shit and should never have been done using CGI - they just dont look real.