ALIENS! is a fixed shooter in which you move your turret along the bottom of the screen and shoot aliens that descend from the top. If an alien reaches the bottom, it's game over. The game starts out easy, but as the game progresses, the aliens get faster and faster. The controls are fluid and the scrolling is smooth. This is a solid (and slightly addicting) arcade game.
The keyboard controls won't work on later models of the PET. I haven't tried disassembling the program, but I suspect that the keboard is read through memory location 151 ($97), which was a frequent cause of compatibility issues.