As I recall, when Episode III came out everyone was looking forward to the conclusion of the prequel trilogy. Mainly because after the first 2 movies, they really wanted to see the franchise they knew and loved stop being screwed with. However, it's (blog post) also widely accepted that Revenge of the Sith is the (blog post) best of the bunch when it comes to the prequels. The big thing that got everyone to go to the theater and watch it though? The wookie army.. and.. yeah..
Source: Star Wars Episode III: Revenge of the Sith Movie Review - Decker Shado