I'm glad this thread was bumped. I think the game's improved a lot since it started. If you have money to shell out for the DLC, there are two new ones coming out (one involving the Seven Wonders of the World and probably Sumeria, the other involving Korea). If not, I think the game is solid these days. When they started, I think they removed some of Civ4's strategies (involving religion, etc), but didn't really replace them. Now they've found good synergy with the social policy system, city-states, and new small wonders they've added with patches.
I still think Civ4 is the best strategy game ever, but Civ5 is quite a lot of fun. The combat is heads and shoulders above and the production value with the uniqueness of each civ (including leaders, audio, as well as play style) is quite good.