I've played Civilization faithfully for years, Master of Orion, Alpha Centauri, etc. I can say with satisfaction that this is one of the most fun and (I daresay more importantly) engrossing strategy games I've ever played. I honestly can't find a single thing wrong with it. There's nothing frustrating or tedious about it, yet it’s so deep and there are so many potential strategies to employ that it boggles the mind.
If I have a lone complaint, it is - as I'm sure it is for others as well - the lack of online multiplayer. However, I fully understand that, as the FAQ says, turn-based 4x games and online gameplay haven't always "gotten along." Civ didn't have reasonably functional online until its third incarnation, and it took until its fourth for it to become more or less hassle-free.
In addition, though I can't wait to someday (as I'm sure it will happen) play online in GalCiv 3 or some later version of the series, I have to say that it's refreshing to see a game live of die on its single player quality in an era when online gaming sites seem to review based almost entirely on multiplayer or the lack thereof. I can't count the number of times in the last five or six years that I've seen a review summary sound something like this: "Despite a lackluster single player campaign, the multiplayer is so much fun and so feature-packed that it shouldn't matter!" Word to developers everywhere; please take note: it DOES matter.