Perhaps one of the best games in ever created across all genres.
I don't know what it is, but unlike any other game I keep coming back to this over and over again. I still haven't won any victories on any setting beyond like prince, but it's still a lot of fun.
The in game permutations of possible outcomes and scenarios I think puts it more on a level with classic strategy games like chess and go. I can easily see wanting to play this game several years down the road.
The BtS expansion is the best IMO. Colonization gets all the accolades, but I found this one the hardest to get into.
I'm waiting for a price drop on CivV, but haven't played more than the demo on this more recent version. I didn't like the demo as much as CivIV, but really need to spend more time with it to make a conclusive comparison of the two.
Everyone with even a remote interest in TBS should own and appreciate this game.