The Great Gatsby is definitely better as a book than a film. It's the way it's written that makes is just "so". I do remember reading the book a first and second time and thinking "is that it?" But it is much more than just a "sad story", there is a deep cautionary tale amongst the frivolity and vanity of the characters and setting.
The film does not capture the wonderful quotes from the book and to me, it interprets it in a different way, not always good. Overall it stuck closely to the plot but I found Baz Lurhman's direction of it too "Baz Lurhman" iyswim. Very camp, flamboyant and blingy.
I thought that Carey Mulligan portrayed Daisy very well, innocent but foolish and selfish. Toby Maguire was also very good as Nick Carraway, he seems to be very good at portraying naive but generally good and loyal type. I thought Leo Decaprio over acted at times especially when Gatsby was first introduced but I guess that was his intention? He and Toby had good chemistry as Nick and Gatsby respectively but I read that they're also good friends in real life which helps.