I would've given it around an 8 at the time it came out.  Now it's harder to judge.

Tbh, I think the "Halo-killer" hype that OPM thrust on the Geurrilla really hurt the game.  It had it's flaws, but it was still a pretty good game, and I think reviewers may have felt really let down after all the hype, and reviewed the game as such.

Edit: If you go to GameSpot, you'll see that the user average is an 8.2, while actual review is a 6.9.

If i would have to "review the game" (I haven't finished it though. But by my current impressions, it would be something along the lines off..

(doing this IGN style)

Audio: 6.8 out of 10.
Lipsync and voice acting works well. It gets tiresome to hear the same few lines over and over again though. When someone speaks their voice tends to dissapear if you turn your back to them. Even though i have it setup as "stereo" As said previously the feeling of war is sort of dim (once you kill a wave of enemys) and it needs more ambience.. and epic music like the music in the menu :P

Graphics: 9 out of 10.
The game looks stunning for a playstation 2 title. Even though it's often not fought in huge locals, some of them actually are pretty big. The game has animation work that can rival new games coming out today and at times there is even like a sort of "motion blur" when you run.

Gameplay: 8 out of 10
The gameplay works, i've remapped some controls, but you can do that however you wish. Luckely. It's a standard shooter but what it does, it does well. The only drawbacks i have, are the "on rails" (although it's not that bad most of the time) and it can be hard to melee someone sometimes, i would also really have liked a jump button, but that's just nitpicking. It could also use a cover system.. fortunately killzone 2 will give it that. the AI is also pretty good. Atleast if you go for a more close orientated battle like i like too.
Another great thing is the different chars you play as. It really mixes it up and makes the gameplay a lot more variated than it would have been had you just played as one. Some people call it a bad thing but i really think it adds to the game. I especially like "rico" who has a big chain gun with a "rocket launcher" thingy underneath. And the assassin (can't remember her name) who has a silenced sub machinegun and a knife so you can run up and cut people.

Presentation: 8.5 out of 10
The game opens with a big cutscene full of epic and win, the menus are easy to navigate (except you can't press start to exit the pause menu) Hud is easy on the eyes and overall it presents itself really well.

Lasting Appeal: 8 out of 10
Well i've only played the singeplayer so far. It seems like it was made so that you easily can go back and play your favorate parts again. The multiplayer is both online and offline And with bots. So in either way you are probably set to play for a long time before it gets boring.

That's my opinion on it :p Hope it's of any use.

i loved the firt one,most loved the multiplayer its was awesome