Anyone complaining about difficulty is just a whiner, the game looks, sounds, and plays fantastic. The levels are diverse and well designed, and as you play you develop the twitch shooting you need to beat the levels. It's fast paced, tactical, and difficult as hell, but you respawn so quickly it doesn't matter. It's not rage quit frustrating, it's "I have to play this level one more time" every time you die frustrating. And the sound design and artwork is flawless. In short it's absolutely worth buying.
