When Civ IV came out it was incredibly buggy but you could tell that there was an amazing game buried under all the bugs. I don't get that feeling with Civ V. It is a nice game and I enjoy playing it but it isn't the quantum leap that II, III and IV were. It kind of reminds me of Call to Power in that regard.
I have been playing Civ V since I got it a week ago but I am thinking about playing Civ IV again soon.