So after playing the beta and now the single-player demo I think I've decided to buy KZ3. The big question now, should I trek through the second one to get some context ? I played it abit about a year ago, but with no extra motivation to push me through it it just didn't really connect for me. The gameplay just was abit too annoying, and with no connection to the setting or characters there wasn't really anything special there. I think I'm primarily gonna get KZ3 for the MP though, but the SP seems somewhat entertaining too.

Just read the plot summary on Wikipedia.

I was wondering this as well...

ISA = good guys (although they really aren't), Helghast = bad guys (although they really aren't). ISA troops are trapped on Helghan and facing a Helgan surprise counter-attack. Rico is an idiot. That's all you need to know. 
 
The story is pretty much awful - as awful as in nearly all modern shooters.

I wouldn't bother if it were me.  Game wasn't bad but it doesn't need to be played to play Killzone 3 I would imagine.  Just think of Killzone 3 but with controls that make the game more difficult to play.  K2 was more about the MP for me tho; as will Killzone 3 most likely.

I think it's worth it, having nearly finished my second play through.  The artificial intelligence is fun and challenging, the battles are intense...it's definitely worth greatest hits.

I just watched a video of all of the cut scenes yesterday. Glad I did because I kinda enjoyed it, and now I know what's going on.

I'd say get it. I picked it up for $20 a couple weeks ago and had a pretty good time playing through.

I think it's on the greatest hits list so it should be cheap now.  Might as well pick it up.

I say yes, it's great fun, yeah, storyline isn't epic, but the game screams polish and quality and is well worth a playthru.