Regardless of the flaws in his films, Lucas at least _tries_ to tell a good story with his films (which are Eps 1, 2, 3 and 4 -- eps 5 and 6 were directed by Irvin Kershner and Richard Marquand respectively); and as you said, they're treated as different chapters within the same story. Lucas makes mistakes, but he takes them onboard and makes an effort to improve on some of them. While Lucas uses loads of special effects, he's ultimately still trying to use them as a tool to tell a story, rather than making stories to tell special effects.

...which is more than what I can say for Michael Bay's work on Transformers.