I've been playing this game for awhile now, as I'm lucky enough to be in the industry. This game should not be compared to Halo, as the only simularity being they are both FPS.
What this game has done is remind me of the power of games. The power to let me escape reality and enter a world from someone else's imagination.
Simply put, Killzone put me into a war in the future. From the graphic environments of debri and dust, to the silent stalkings of the enemy, I felt like In was in a real war.
The game allows you to control 4 different characters through 11 levels. Over 25 different weapons are available.
This game screams production value, everything from visuals to sound design are top notch. Let us not forget this game was designed specifically for the PS2 - and what a design this was.
It's not Halo. It's war. Dirty, gritty, and well ... fun.