The game seems to work fine on my older 333mHz G3, with no real video acceleration -- but i would HIGHLY recommend changing from the default 800x600 to 1024x768(?), otherwise you cant see diddly in the menus on the right side. The info/scenario screen still run in 800x600, and i'm getting a grey bar on the right and bottom of the screen, but it's not that annoying.

I tried it in the Classic-enviroment, but it had some problems with the screen. I couldn't find the right resolution. There was always a piece of the game I just coudln't see on the screen.

Living in Germany I've had this game for months already.
I can't use it anymore since it does not run under OSX when you do not have a classic environment installed.

I got Alien Nations from Mac-o-Rama. I've had it for months. They seems to distribute a lot of European-made game. Fun game!
Jude

This game has quite low system requirements, and would probably perform quite well running in Classic. Why not download the demo and give it a try?

http://www.macgamefiles.com/detail.taf?item=16928

...but there is no OS X Version as Jonas said.

I bought it about two months ago. The game is great, but there is no MacOSX-version. :(