A good expansion pack overall!
The 'caretaker' pregenitor faction seeks to prevent Alpha Centauri or Manifold Six being exploited and halting the forcing of transcendent evolution, the 'usurpers' meanwhile seek to exploit manifold six and both are locked in a civil war.
However as shown in the opening cinematic both sides are stalemated and destroy each others main ships but crash land on Chiron like the humans in escape pods and have to rebuild from scratch and rediscover their technology.
Unlike human factions the Pregenitors can choose to research much more precisely what they want than the human factions which under a skilled leader can make them incredibly powerful if they go for the right technologies when they are avaliable. And they tie into the storyline since they were present on Chiron first (having left monoliths around) and when first contact is made neither side can communicate with each other until the techonolgy is researched allowing for communications to take place.
Also if a player meets on alien faction and makes deals with them and then later meets the other alien faction, the latter alien faction will cease diplomatic negotiations with you and dislike your faction but may not necesarrily start a war with you.
One an alien faction is at war with a human one, they are effectively on a war of destruction and each city they take the human populance is killed and most of it escapes leaving a 1 pop city behind. Likewise when a human faction is at war with an alien one and takes a alien city, the same happens.
Also both the alien factions are always locked in a war (vendetta state) with each other.
Furthermore the Pregenitors have a unique winning condition unavaliable to the human factions, were they build a Resonance communicator and call in massive reinforcements to overwhelm all opposition and achieve total victory.
But its not just alien factions that were added, a few more human factions were too, the nautilus pirates are a faction that actually land in the ocean and are much more suited to life at sea than on land.
Cult of Planet is a mix of the Gains and believers I feel, a cult obsessed with the preservation of Chirons lifeforms and envrionment. Cybernetic Consciousness is a faction that is actually cybernetic with a lobotomised brain and a harddrive.
Interestingly the consciousness can download a technology from a rival from every base it captures of theres, and it can be freindly even late game with a faction until it has the option to take cybernetic future society in the social engineering tab and the player refuses to do.
The Freedrones are those oppressed workers seeking to create a better hardworking and fairer society.
And finally there are the Data Angels, who are a free for all anarchist group which specialises in hacking and seek a freeflow of information.
Other additions to the game included a few fixes to the alpha centauri game play, prototypes actually display the cost correctly in the addon.
Also there are new unit additions in the form of weapons and abilities for the player to exploit.
And theres a few new buildings and secret projects complete with their own high quality cinematics offcourse.
Those are the differences to the game all of which I thought were quite positive, who doesnt like new factions and buildings in a expansion pack?
I also thought it was a good idea to keep the play on alpha centauri itself, they created one hell of a good futuristic sci fi and alien setting in the original game why abandon it?
The alien factions though sometimes feel overpowered, they have combat bonuses and research is made easier but they are good additions to the game and help to spice the game up.
Its such a shame though that this had a limited print and all of it was sold out so the game is exceedingly rare these days and fetches a high price on the likes of ebay and what not, so good luck finding it, im so glad i got given the 'planetary' pack which included the original and expansion pack.
Lets hope a second print is made for Alpha Centauri Alien Crossfire.
The original Alpha Centauri is still widely avaliable at budget price these days so go for that, and then maybe save up the pennies for the expansion pack.
edit: update as of march 2009
Turns out 'sold out software' a publisher in the UK which sells low pricebudget games has now released a 'Alpha Centuari Complete' Pack for 5 pounds which is fantastic news!