A good game that falls short of it's hype and merely is a mediocre experience.
In terms of a story this game has one but it sure is underutilized and if you swapped the order of the levels around no one could tell the difference because they are pretty much the same set piece battles in a different environment that do nothing to tell you what is happening. The main plot is that the Helghast, a mutated race of humans, invade a earth colony called Vekta to basically try and take over the Inter-Strategic Alliance, or ISA for short. You are a specialist that is tasked with a mission to help repel the Helghan invasion and you and your team of soldiers have to mow down hordes of Helghast in order to achieve this goal.
The first thing that you will notice when starting the game and moving your character is the amount of detail that went into how your character moves. Every time you take a step the camera jumbles to give you feedback that yes, you are moving and have gear strapped to you that gives you a heavy gait. Weapon animations are very realistic and reloading is some of the best animations I have seen on a PS2 game and they really give you a sense that reloading in the heat of a firefight is no walk in the park. Also the accuracy of the weapons is pretty realistic as well, maybe a little too realistic because trying to hit an enemy more than 100 feet away takes deliberate aiming and burst fire that feels like a little too much work. The reason I say the realism is bad is even though you have a team of soldiers that tag along with you, that is about all the work that they will do to help you, just tag along. Your A.I. partners will never take the point in a fire fight and rarely ever even shoot at the enemy, they seem more comfortable to watch you and shout encouraging statements when you manage to land a head-shot or blow up a tank. If your squad mates actually employed squad tactics this lack of video-game accuracy in the weapons would be excusable.
Back on to your A.I. teammates, one of the neat parts of the game is you can control each of the teammates that follow behind you and play using a different style for any of the levels. You start off as a specialist that is a jack-of-all-trades with weapons but you have a partner that specializes in stealth, a guy that uses heavy weapons very well and a spy that is trained to use Helghan weaponry better than your other squadmates. This gives the game a little replay value because the characters do play differently and the story branches off the main paths in some sequences to highlight the selected character's strengths. But the storyline and the voice acting done by the characters is very weak, you will find yourself laughing at dialogue that is meant to be taken seriously but the hammyness of it is just too much so you might find yourself deciding that one play-through is enough.
The helghast on the other hand are pretty good opponents that don't just stand in one spot and allow you to pick each one off like you are at a turkey shoot. They find cover, throw grenades and are tough as nails to kill. Average soldiers can sometimes take up to ten bullets to be put down and more powerful foes later in the game can even stand up to twenty on the normal difficulty setting so running and gunning is not advised since you are about as strong as they are. Your health regenerates over time and this is displayed on your standard issue health bar and you can also use med-packs strewn throughout the levels as well. The game does force you to use cover mechanics and while this can be frustrating at some points, when you do clear a level you feel a sense of accomplishment that you just wiped out dozens of Helghast and managed to survive. And by the end of the game when you finish the last mission without dying you realize that this game can have some fun points even though it is a derivative shooter.
As far as online multiplayer was concerned, I was actually not able to jump on due to the age of the title. So my opinion of the replay value of the game now is pretty low considering there is not much to do after you beat the campaign using every one of your squad's characters. And since the average play-through will probably last 12-20 hours then you can see that if you can't find this game for a budget price it does want warrant a purchase.
Overall Killzone is a good shooter that does everything well but nothing fantastic or new. My overall recommendation is to rent the game first and if you really like the almost non-existent story or just love hearing bad voice acting then you might consider buying this for your collection. But this is definitely worth at least a look from anyone that would like some back story filler on the second game and another set of Helghast to blow away.