Just to balance the Conquer the World comments, I find it quite intense and addictive in the way that SMAC used to be. Once you start taking over provinces and the AI counter-attacks, you'd better be on your mettle. It has a more Civ feel than the rest of the game, because you have to pay attention to your provinces that don't have armies based in them. I'm looking forward to trying some of the specialised scenarios as Napoleon or one of the nations being invaded by European colonists.
There are other CTW scenarios out there, including one which adds a further 23 provinces and another Middle Earth (as usual) version. Now, if someone could just make an Alpha Centauri one ...