Frustrating and difficult at times. But the game still manages to be fun and addictive.
The graphics are surprisingly solid (mainly because of the time when it was released) but you can suffer from glitches during scenes and gameplay. These are noticeable but minor so it's not much to be worried about. Voice acting is good.
The gameplay is the winner in this game. Tight and solid. It's action-packed and strategy is very important while playing. The CPU delivers smart attacks and tricky defense positions, which it can bring frustration and difficulty at times. Bosses will give you a huge amount of frustration, probably because they will kick your butt hard for about 3 or 4 times. You WILL die, and I do mean a LOT. Even if you're on Easy Mode, there's no way to evade this. Unless you turn out to be a real pro. You will have different weapons to choose and you can upgrade these weapons if you don't forget to collect the money while fighting the baddies.
A sad thing about the game is that the campaign is kinda short. But, the challenge games are there to entertain you after you've finished the campaign. So, I say you could finish this game in a couple of weeks. That is, if you are a slow gamer like myself.
In the end, Killzone: Liberation is worth playing and I definitely recommend you to get it cheap and have fun with it. But I warn you again, be patient. It ain't as easy as it looks. But who doesn't like a challenge now and then?