A very good FPS and for some reason very unfairly reviewed.
To start off, the game is beautiful. It just looks amazing and there will be several places where you'll just think "wow". The game really gives a realistic feel of a war zone, especially with the excellent sound design. The frame rate does slow down occasionally but not to an extent that kills the gameplay, and its only momentarily. The graphics and sound of the game provide a great atmosphere.
Onto the gameplay, the controls are good and give a "weighty" feel to the game and they work very nicely with the PS2 controller. You feel like a soldier lugging your equipment around. This is not a bad thing, it feels realistic.
The game is quite slow paced but not boring. You can't just run out into the open during a fight and hold down fire.. you'll need to take cover a pick your shots. The enemy will do the same meaning firefights can take some time.. The AI is not bad at all, the Helgast will always seek out available cover and nail you if you carelessly step out into the open ground. You'll find yourself often pinned down by enemy fire and having to dart between cover in order to get a better shot on somebody. In the main story you'll soon be leading a squad of 3 other characters who will follow you provide support, you'll always "take point" but when hell breaks loose they'll step in and help out. A big full scale battle in Killzone is just a lot of fun to play.
The other mode (Battlefields) is basically an arena based FPS game with deathmatches, assault, capture the flag (supply drop) modes.. some defend the base type things. This is also a lot of fun to play and carries over the campaign mode's style of gameplay. It works well although some levels are better than others. Two player split screen is good although the framerate does suffer.
The story mode is quite long, and you can play it with 4 different characters. The story itself is good if not particularly complex.. but it moves the game along nicely.
The game does have a couple of problems, the framerate as I mentioned earlier is not really a massive problem but it can be annoying when it happens. The game's sound has a couple of bugs like certain gun noises will stop working very occasionally (which kills the atmosphere). Also, playing as the character Luger is just pointless, she's meant to have stealth skills but the game is designed for big shootouts.. so she just isn't very fun to play as compared to the others.
Overall this is a really good FPS with a good story, fun gameplay and amazing graphics and sound. The problems don't even near spoil the game and I believe it has been very unfairly reviewed for whatever reason.