It's just as fun as the newer entries in the Civ series, but in space. You can see a lot of the ideas that went on to be implemented in Civ3 and 4.
My favorite part of AC are the unique leader personalities. Lal is my favorite personality-wise, because he is a crazy motherfucker. My favorite faction to play as, though, would be University, as I heart researching everybody into the dust and then pounding them into dust. (As usual).
The unit customization is something I haven't yet seen in any other Civ game. It rules a bunch, I must say. When I've maxed out all techs and I have the space elevator, I like to create a unit with maximum attack, defense, drop jets, and an uber fast reactor, and call it the Juggernaut. With it, I can strike any land-bound city in the game (except when it has air defenses, even when there's no damn airplanes in the city). I usually only need one unit to take the city.