An excellent FPS that you'll enjoy, especially if you're looking for a bit of a challenge.
Killzone 2 suffers a little with controls. Looking around and aiming feels ponderous, even with high sensitivity, so they might take a little time to get used to.
The single-player campaign is quite fun. Various types of enemies are introduces, some bosses show up, keeping the campaign interesting. It might not sport the best story or have a memorable cast, but it will keep you entertained, and in some instances, lets you run and kill things with impunity, and who doesn't want to do that? The game gets two of these moments. One is when you're driving a huge, metal mech-type thing, with infinite bullets, and infinite missiles, and another is when you take one of the bosses's lightning gun and wreak havoc with it.
There is only one moment, near the end, when the game gets frustrating. A group of enemies with rocket launchers come out to face you, and you know how it is with rocket launchers. (If you're screaming at this moment, I suggest you take out the knife and run like hell in their direction, since the knife kills in one hit and running like hell makes them miss.)
The AI, both enemy and friendly, is pretty good, and you won't find yourself frustrated with their useless repeated deaths too much. The enemy Helghasth will employ various tactics to get you. They rush you with submachine guns, use turrets to take you out from afar, duck beneath cover until you stop shooting them and even spray blindly if they think you're approaching. They'll also throw grenades if you're being overly cautious behind cover and try and knife you if you get too personal. Despite being an AI that can respond in any situation, they don't have a personal grudge against you, so, more often than not, you'll find yourself safe and sound while your teammates get fired upon.
Killzone 2 also has a multiplayer. It sports the same control flaws as the rest of the game, but it's probably one of the best multiplayers I've ever played. Firstly, the multiplayer rewards you every step of the way. You get ribbons and medals for doing all sorts of challenges, and these eventually get you weapons and classes. You start out with no class abilities and the trustworthy M82 for a gun, but eventually you'll unlock a shotgun, a couple of sniper-rifles, a few rocket launchers and a sub machine gun. The weapon selection is rather limited, but all the guns are good and fun to use.
The class abilities, which you unlock along with the guns, give the players a few tactical advantages. Different classes can hack machine gun turrets and repair ammo dispensers. They can deploy drones that patrol an area or a small turret that will keep the enemy at bay. Medics heal and revive teammates. If you're more of an individual player, the game still satisfies. Snipers can turn invisible and can serve the double-function as long range support and spotter for the team, thanks to one its abilities. The Sabotage class allows you to disguise yourself as an enemy team member. You can be detected, but if you play it smart, a couple of C4 charges will take care of the enemy. And even if you want to run around and blow stuff up, there's a class with body armour, rocket launchers and an ability to increase running speed.
There are various modes, namely the ones you'd find in typical first-person shooters. These are Bodycount (a regular team vs team deathmatch), Search and Destroy (team defends area, other team tries to blow it up), Capture and Hold (regular domination match), Search and Retrieve (capture the flag, but with a propaganda speaker) and Assassination (kill a specified member of the enemy team, or defend a teammate that the other team is trying to kill.) While being quite regular, they're all still fun, and they're made even better by the huge, intrinsic maps. They are widely different, offering varied gameplay. You may find yourself fighting over the highest level of a bridge in Corinth's Crossing, trying to to gain a point from which you can fire on their spawn, or you may be at the lowest level of the bridge, traversing narrow walkways and trying to overcome chokepoints, or holding them. You may find yourself in Radec's Academy, fighting in large rooms and defending tunnels and ladders, shooting across the great courtyard in the middle. Or you could be in Salamun's Market, a somewhat urban level with buildings and streets, or in a desert area with the occasional ruined building and vast gulfs, crossed by bridges that serve as chokepoints and a shooting gallery for the good sniper.
You never lose the feeling that you are in a war, Various effects, like flying mortar fire and the sound of artillery fire, the lightning flashes across the sky, which may not be necessarily natural, and the sound of the narrator (the voice that speaks up in most FPS, telling you whether you are close to victory and so on) demanding that you take down the "ISA dogs" make you feel as part of a faction. This is heightened even more by the propaganda speakers you find for the capture-the-flag style matches, which constantly play speeches by Visari and whoever is the leader of the good guys.
And if you have a bad internet connection that might put you off the multipalyer aspect of the game, there's a Skirmish mode that allows you to set up "multiplayer" games with up to 15 AI bots, and you don't have to rank up to use all the weapons. You can still enjoy it in a way, and since the AI has several difficulty levels and isn't bad at all, it will be challenging and fun.
So, if you're looking for a brilliant FPS, Killzone 2 is what you need to get.