AI Behavior in a Space Shooting Game
My first real game that I made (ie: post-pong) was a 2d space-invaders clone like the one you describe, and it's ai was pretty simple. basically the enemys were trying to reach the bottom of the screen; if they got past you, they won.

Basically I think it's smart to use a variation of predertimend AI behaviors without including much randomness at all.
thats what I did

When they spawned, they randomly chose one of 5 'ai styles':
-try to attack the player, angleling (sp?) down towards him
-rushing towards the bottom in the middle, shooting constantly
-rushing towards the bottom on the sides, shooting constantly
-camping up the top of the screen, trying to snipe the player from afar
-and one other one, which i've forgotten

These styles were pretty basic, but as you were facing 40+ of them in the later levels, it looked pretty impressive

With 20/20 hindsight, there were probably far better ways of doing this; I could have made them team up (ie: they wait up the top, then attack in waves) and make them try not to shoot each other so much .
Formations would have been cool as well...and having them change attack styles if losing, or if the player is low on health / occupied...or all rush for the side of the screen the player is furthest away from...damn, why didn't I think of this stuff at the time?



...whew, that went on a bit longer than I intended