Sid Meier, one of the best designers in gaming industry in my opinion, has done it again. It's true that Alpha Centauri looks, feels and plays a lot like Civ games and that's the real beauty of it.
One million alien visitors from another star system could already be lurking in the solar system. We aren't talking about "little green men" here, however — more "little (and not so little) gray rocks ...
This is better than any of the Civ games, IMO. The only turn-based strategy on PC that competes with it is Master of Orion 2.