Absolutely love Alpha Centauri, i might even have those big tech tree posters somewhere in the basement.
As i recall i kind of liked Alien Crossfire, apart from the ridiculous alien factions (and the broken-as-hell Cybernetic Consciousness). The Pulse/resonance weapons and armor expand the rock/paper/scissors aspect somewhat and the new sea resource buildings make sea bases more viable. New factions are a bit uninteresting, although i tend to like Domai and his Free Drones the most.
The new planetary fauna can go die in a fucking fire though, fungal towers in particular. Spore Launchers are fine i guess, they are more on the annoying part of the scale, compared to the ungodly menace of towers.