I will put it to you this way. From around 2000 on, SMAC or SMAX has been on my hard drive. I always re install it and always go back, even though sometimes I may get tired of it and let it rest from time to time. Most of the folks vocal about it say the same thing.
This is even with some pretty stout heavy hitter games on my hard drive.
Only thing which may be a turn off is some folks do not get into transhuman sci-fi. The game is also complex even by Civ standards and was a fair critisicism of it, however I think it is a plus. Graphics at least are not Commodore 64 bad or worse ASCII, but newer gamers raised on lots of eye candy may be turned off. However, if that can be gotten past, the gameplay has not been matched even with modern 4x.
Oh yeah, SMAC/X also has storyline! It is like a novel.
Sad, they never really did aything with the franchise. This, along with some other games, are considered cult classic.