Every expansion for AI War is a must have (as is the base game, of course), but this one is an absolute star. It seems to lack the single "killer feature" of every other expansion, but then every one of this expansion's features offers a great addition to the game.
Dual-type AIs are, to me, the best bit, and I could never want to go back to single AI-types again. The ability to double up the AIs tricks on each AI provides opportunities for nigh-unbeatable pain. Speed-racer/Artillerist? Of course! Shadow Master/Grav Driller? Perfect!
The new AI types themselves are a good selection, including the newbs-favourite, Chivalric, many other people's favourite, Bouncer, and one which everyone fears, Vicious Exotic. And a bunch more.
The new AI plots are worth a look, and it's rare that I start a new game without the Hunter sub-plot enabled. Yes, I really do think it's fun to have hunter-killers flying around the galaxy as a matter of course. Don't you?
The new core guard posts are, of course, usually only an issue in the latter stages of the game, and you won't always get there, but they really do offer some excrutiating, sorry, exceptional challenges to trip you over in the latter stages. You're going to lose ships. Make a mistake and you're going to lose lots of ships.
The Dire Guardians are another excellent addition. Spawning from Dire Guardian Lairs, they add extra fear, extra danger, and extra pressure when you see the warning that one is being built. Better go and take out the Lair? Of course! But bring an army because when it pops, three of the blighters will emerge, armed and angry.
AI Subcommanders? You'll wonder where they are, and even what they are until you meet them. Wait 'til you meet the Rude Gesture subcommander. Wait 'til you try to take out the Rude Gesture subcommander!
The Core Turret Controllers add a whole new aspect to the hacking and defence games. Need an extra powerful layer of defence on your whipping boy (perhaps to fend off that Vicious Exotic? You want these controllers! You NEED these controllers!
New ships include the Lightning Torpedo Frigate, powerful, but with definite weaknesses, and the Protector Starship, which if handled correctly, can essentially neuter superfortresses and (some) core guard posts. You want these too.
Oh, and if all the rest wasn't enough, there's a whole new means of achieving victory. In the unlikely chance you live that long.