Since it doesn't clearly state anywhere on the box, let me make it abundantly clear that Knife Edge isn't really a "fly-around-and-shoot-stuff" helicopter game. It's really just about shooting enemy ships while the computer flies the helicopter around for you. If you can remember Pyramid Patrol or Sewer Shark, you know exactly what kind of game it is: a 3D railed shooter.
Knife Edge has a incredibly cheesy story to it. Get this. Earth colonies on Mars have been invaded by insidious alien ectoplasm. You, of course, being the gunner of the experimental assault aircraft known as Knife Edge, must shoot 'em all before they take over the planet. How do you this? Simply line up the crosshairs with the enemy on the screen and shoot. Unfortunately, this is pretty much all you really do for six levels (OK, you can fire secondary weapons and dodge enemy fire, too).
Visually, Knife Edge is pretty mediocre. The majority of the game is composed of dark, drab, uninteresting environments and enemies, although admittedly some of the end-level bosses are pretty cool looking. Knife Edge doesn't offer much on the audio side either. There are little techno beats and bad sound effects, but no voice-overs or anything especially neat. The only thing about Knife Edge that is sort of cool is its multiplayer mode, in which you can play the missions with up to four people at once. Since the gameplay is so basic, it's a perfect game to rent when you have friends and family over who might not be expert gamers.
The game works well as a rental, but since you can play through it in just a couple hours, I advise against purchasing it outright. The developers tried to extend the game's length by adding alternate paths, but it doesn't add a lot. Once you've gone through the game a couple times and seen all the cool bosses, you'll be done with it.