I need to reassess this book. The original version of my post was under a misapprehension about what it was and what it was doing, but after quickly popping in to the video game version of Sid Meier's Alpha Centauri, I realized GURPS: Sid Meier's Alpha Centauri was a much different beast than I'd originally made it out to be.
See, the problem is that I have a lot of good memories of SMAC, so I was inclined to be very generous about its merits, but a corollary to that is that the more generous I was to the video game, the more I was tempted to discount the unique contributions of the book.
The thing I remember very clearly about Alpha Centauri the video game is that it had a rich and fascinating back story, and when I read the rpg adaptation, my primary feeling was "none of this surprises me." So, I guess I just assumed that everything that was good about the book was present in the original game.
The turn around came when I was writing my first post's Ukss contribution. I went with The Dream Twister, and was a little frustrated with how little detail the book had about it. So I decided I would look up its entry in the game's Datalinks, and use that that to flesh out my entry. Only, when I did that, I discovered that the Datalinks entry was just a couple of sentences and, in fact, the book went into dramatically more detail.
It was a very weird feeling. Everyone knows that Sid Meier's Alpha Centauri is the gold standard when it comes to 4X games with a plot. And make no mistake, it's still so great that I very nearly got sucked in to playing it all over again, but it's shocking how much it is just roughly sketched and left to your imagination to fill in. The Dream Twister has this creepy, evocative video that absolutely sells what a terrifying psionic weapon it is, and it's so effective that you never really notice that the game never comes out and tells you that's what it is.
So I'm not sure I can really fault GURPS: Sid Meier's Alpha Centauri for being vague. What I wanted was a detailed write-up of The Dream Twister as a location. What would it be like to set an adventure there? What horrors would the PCs witness as they walked its halls? What sounds would take up permanent residence in their nightmares? The book answers precisely none of those questions, but it does give us three paragraphs of social and technological context, which comes to about six times the information in the video game.
All-in-all, the book is a slim volume which doesn't quite provide enough useful information to run an rpg. There's more original detail than I gave it credit for, but it really is quite conservative in its extrapolations. It never commits to building a concrete Alpha Centauri canon, and as a consequence often comes across as pointlessly vague (it's filled with things like, "the faction which built this device . . ."). So it never really gives you the benefit of immediately gameable scenario ideas like, "the turmoil that resulted when Morgan Industries monopolized the Longevity Vaccine." Which is a definite weakness. The video game needed to keep its options open, but when you try to put things into a narrative, you've got to make choices.
Ultimately, GURPS: Sid Meier's Alpha Centauri feels like a curiosity. It's a fun little illustrated guide to the game's backstory, and it was an absolute blast to experience the world again in a new form, but if you don't have the benefit of nostalgia, I'm not sure it makes a very compelling case as to why you'd want to use it as the basis for a sci-fi rpg.
Ukss Contribution: I'm really spoiled for choice here, because SMAC has a lot of bold science fiction ideas, but I'm going to stick with my choice from the first version of the post. The Dream Twister is a creepy psychic WMD, and I'm just going to have to come up with the ground-eye view myself.