Ok, I bought it, cause I got bored. Overall, it's not a bad game, it's just not that great either. It has some nice features in some cases, and then a lot of points that are just weak.
The AI is basically just SLOW. They'll run for cover, but then they'll just kind of stand there for a while not doing anything. Then don't react very quickly, so much of the time, you don't even have to take cover, you can just stand out in the open and gun everyone down. Of course, eventually, they throw so many guys at you at once that atlreast a few guys will be shooting at you at all times, but it's just a matter of using cover the block the guys that are shooting at you while you kill the guys that are asleep. Overall, they throw enough guys at you that it's not too bad, but it's still not really hard in the slightest.
The weapons are mostly unremarkable. The basic assault rifle that you start with is pretty good, because it has first shot accuracy and recoil, but almost all the other weapons just have static conefire spread that doesn't change at all. Annoyingly, they don't give you a lot of ammo for your assault rifle, so you have to use the enemy assault rifle a lot, which doesn't have any first shot accuracy, so you have to resort to spraying and praying, and occasionally using the buckshot alternate fire.
Multiplayer actually DOES work offline with bots and split screen, so I played with my sister a little. Unfortunately, the bots have a ridiculous amount of health in multiplayer. In single player most of them have the same amount of health as you do, and will go down with 2-3 headshots or 4-6 bodyshots, but in multiplayer the bots seem to take entire magazines point-blank without dieing. At one point I was up on a sniper tower shooting at guys with my assault rifle, they were just standing around in the open easy targets getting shot, and I was shooting with accurate single shots, but I used my entire 300 rounds of magazines and only made 3 or 4 kills. This makes bot play extremely boring, because the bots take forever to find you/for you to find because they are so dumb, and then it takes forever to kill them.
Health for players is fine though, so if you had enough people for a good split screen match it would probably be OK. So far I have only tried team deathmatch and domination. Team Deathmatch has random spawn points, which doesn't make a lot of sense for a tactical realism-oriented game. You don't have a radar or arrows either to find your team mates, so even playing split screen you spend most of your time with all your team mates scattered randomly around the map; it's very difficult to co-ordinate. In domination, I haven't even figured out where the control points are yet. They aren't marked with large colorfull icons like in UT, so unless you have played before, you have no idea where to go or what to do.
On the plus side, the maps seem to be pretty cool. Unfortunately, the non-linear, random nature of play kind of brings them down, but there are a several outdoor, very realistic, but moody and stylized outdoor maps. With good gametypes, they would be fun to play around in. There are also some indoor maps I haven't tried, and several other game types.
Anyway, overally it's one of those games that's frustrating because there's parts that are cool and show a lot of potential, but then other parts that are just incomplete or half-assed. I think the direction that the developers TRIED to take the game in is a very solid concept. It's a combination of sci-fi and realism, and of futuristic technology with archaic dirt and grime. A lot of the weapon designs are pretty cool, conceptually and visually, it's just their implementation in game isn't quite right. For example, the enemy "Helgast" assault rifle has a bull-pup design with a 50 round helical magazine, an infra-red scope, and an attached shotgun under the barrel. Unforunately, it has ****ty static accuracy, making it a complete bore to fire, the shotgun only holds one shell, and doesn't seem to pack the real punch of a 40mm buckshot round, so it's not really as cool as it could be. The developers spent a lot of time on making things like reload animations look really realistic, but then they make the oversight of having the player pull the charging handle every time he pulls out the weapon, even though it should allready be ready.
Anyway, it's one of those games that makes you really WANT to like it, but then it just doesn't deliver in enough places so you can't really like it that much. Overall, it's not a bad game and it still probably one of the best FPSs on PS2, but unfortunately that's not saying a lot coming from PC games. It really seems like the game just wasn't finished when they shipped it and they just rushed it out to meet sony's demands. Maybe they will make a sequel that improves on all the undeveloped areas of the first game.
EDIT:
Ok, I figured out Domination and tried Assault. They are much better because the spawns are fixed; each team spawns at a specific point on opposite sides of the map. Also, the objectives make for more concentrated action. The weapon balance is rather quirky though. The basic ISA ("good guy") assault rifle is one of the only 3 weapons in the game with real first shot accuracy, but all the other weapons overpower it with sheer firepower. Why take the time to aim and make carefull accurate shots when you can just spray your heavy machinegun at the enemy's general direction from 200 meters and make the kill 3x faster? On the other hand, the hellghast ("badguy") assault rifle is completely dominant at the 1 or 2 CQB maps in the game because of the undermounted shotgun. The ISA can get a shotgun if they look all over the map for it, but the helghast spawn with it, so i CQB situations they can just insta-kill any enemies (which is very overpowered because most other weapons take atleast 5-6 rounds to kill average).
Overall though, with the exception of the 1 or 2 CQB maps, Domination is pretty fun. The weapons become less of an issue as you can just kill the enemy with more or less anything you have reasonably fast if you get the drop on him or use team work. It's still a little frustrating that you can just anhilate everybody from any range with a super-powered high ROF heavy machinegun with RPG alternate fire, but it's not so bad. It's more about the course you take around the map and how you work with your team than what weapons you use. So multiplayer in Killzone may be pretty fun if you can find a bunch of people to play with you (I don't really feel like getting a broadband adapter or signig up with sony, so right now for me it's just splitscreen).