At the risk of going off-topic, I also don't believe SMAX is worth getting.
The original 7 factions were all based on ideologies that people legitimately believe in (maybe not the Spartans so much - the game doesn't really delve into their ideology and I figure they just needed some sort of gung-ho military faction because every 4X game has one of those). Of the new 5 human factions, the only one I can honestly say fits that description is the Free Drone faction - but even then, it's not led by someone who'd realistically be in charge given the backstory with the Unity and whatnot. And Pirates, yeah - why would a fifth of the people landing on Planet essentially decide, "Alright bros, boats. Boats are how we rebuild civilization." The aliens didn't really have much of purpose - the focus of the game was supposed to be on human interaction with Planet and each other, anyway, which has nothing to do with a third, alien, party.
In terms of gameplay, you'd get bizzare things like Spore Launchers sitting on Isles of the Deep and bombarding your bases, forcing you to build boats when you'd otherwise have no reason to. Or the Battle Ogres in pods - whichever faction finds one first just ends up way ahead. Simply put, there was too much randomness involved. I've put sincere effort into liking SMAX (so I could run the game in something other than 1024x768), but even with the technical benefits of it I can't bring myself to play it over the original SMAC. And this isn't a nostalgia thing for the original SMAC or anything, I got both games at once.
SMAC, though, is a definite must-play. I don't even like the Civilization franchise much, but I'd easily rank SMAC in my top 3 games. Brian Reynolds' philosophy degree really shines through the whole game.