I've played 2, 3, and 4. I think it is a great idea, but one that has to be executed well. From what I've read, the original Civilization was okay, but not that great. It was not until Civilization II that it got really good. Civilization III was a disappointment, IMHO. Civilization 4 is a masterpiece. The only complaint about that game is that it offers so much more in the expansions than in the original game, and that some of the expansions go off in odd directions (space, fantasy realms, etc.)
Overall, I enjoy the series, especially Civ 4; it has Leonard Nemoy as the narrator, has an excellent theme song (Baba Yetu, which is really the Catholic version of the Lord's Prayer sung in Swahili), excellent graphics, and is, on the whole, well balanced and fun.