The first product of Rage software, this game existed largely to make people buy then-novel 3D cards. It's attractive eye-candy, a kind of modern 'Defender' clone with lots of smoke and attractive graphics...
...and that's it. As a technology demo it's excellent, but as a game it's like a blank slate. Whilst there is some variety in the gameplay (you leap from static turrets to roving helicopters and beyond), there is absolutely no depth whatsoever - it's just a target-shooting game. It very quickly becomes extremely tedious, and it's amazing that there was a mission pack.
The Bottom Line
Thrilling eye-candy, no game at all.