Killzone will kill a few hours if you really get into it.
The story was perhaps what was the coolest thing about the game, four guys fighting in different environments looking for a general that betrayed the ISA forces. That was about it, Guns in the game felt a little crappy, they did not have the right feeling, and the way you used them was not very realistic.
Online, Gamespot said was OK, I think it was a little less than OK, with only a few maps a and few game modes, all of which get boring after a while leaves you wanting to go back to story mode and beat the game again, and again. That's the only way you are going to spent time in the game.
Characters in the game are pretty cool, if you want to go nuts shooting everything, you can pick Rico, if you want to be stealthy you can use Luger, she also has night vision, which you will never use throughout the game if you don't want to, but after you unlock Hakka, you are going to be wanting to just play as him because he has the best accuracy, and you could say "unlimited" ammo, since you can carry a lock of ammo from the Helghast dead soldiers.
The online may be a love it or hate it, in my case, not like it, you will find more fun playing in hard mode with the bots rather than playing online.