I just finished it on normal (I forget the killzone name for it) and it was one of the worst campaigns I've played in a shooter.
-Unbalanced. Why am I constantly getting gunned down almost instantly like in a call of duty game on veteran? It wouldn't be a huge issue if each helghast didn't take 100 shots to kill. I don't want to hear "ohhh turn it to easy". No, you balance your game to be not too hard and not too easy on Normal, that's what it's there for.
-bad friendly AI. "sorry Sev I can't reach you". Fuck off Rico, you're 2 feet away from me! Also, why are you shooting a wall?
-bad hit detection. When I'm shooting through a broken car, the only cover in site to stop me from getting 2 shotted again, why are my bullets bouncing off an invisble wall where the windows go?
-buggy. Rico got stuck several times when I needed him to move to access a computer, forcing a restart each time. If you pause during a cutscene, the lip synching becomes as much as 3 seconds off. I saw a grenade get stuck in mid air and teleport forward right into my face.
-too many on rails sections. I thought I was playing Killzone not Time Crisis. I don't mind a few but man they went overboard.
The story wasn't anything special either. It wasn't hard to understand, just completely forgettable. It was your typical over the top save the world with 2-3 super human people BS. I found myself battling the controls too, whether it was staying stuck to cover when I wanted to pull off or trying to get into what the game considers to be cover (crates that are taller than me are not cover apparently, only ones that leave half my body exposed to fire)
It had moments of fun, but it was tarnished by all the bad stuff. I wanted to test out multiplayer too but the campaign was so bad I'm too pissed to play right now.