Alpha Centauri takes Civ II and expands it with some really neat features. Like regarding units, where you can combine techs and make up your own units, or superunits. And with tile improvements, they took advantage of the sci fi setting to do what they eventually did with Civ IV, add a tile improvement for commerce.
The AI is sharper, the Diplomacy was really upgraded, and with it all they added an ongoing "story" to the game where you get more and more "in tune" with the new planet you're on and when you take all of those things together what it means is if you're a fan of Civilization II, III, and IV, then you'll definitely LOVE SMAC.
However, because of all of it's new features, there is some more learning to do. I used to have set strategies on setting cities in II and III (and to a lesser extent IV) and thos strategies had to be thrown out the window and completely relearned for this one.
For example, there are ways to get resources for your city without workers. Lots of ways. If you've ever felt you had the right "mix" between food and commerce and production figured out in Civ II, III, or IV, they have to be completely relearned here because with those features, it's almost limitless what you can do with cities (or sort of, the unhappiness levels will keep you in check).
Anyway, it's sort of like Civ II on steroids, with better AI, better diplomacy, much more challenging barbarians (here they're called mind worms and stuff like that and they will mess you well into the game unlike others in the series), and way, way more options.
I won't go into the expansion, because that's currently not offered here and there's definitely two schoools of thought on that. One decidedly positive, one negative.
In short, if you enjoyed Civ II, and Civ IV (without the resources but with way more other new options) then I think you'll love SMAC. Once you've played through a couple of times and get a feel for all the changes.
EDIT: Oops, TLTR. I'm bad about that.
Post edited May 18, 2012 by OldFatGuy