In its current state, Killzone is broken. Multiplayer suffers from the worst lag I have ever seen since the invention of the internet, (up to 30 seconds of lag). Visit the KZ official forums or youtube for more info.
The game wasn't that much fun to begin with (no where near as good as KZ2), but at least it was playable. Somewhere along the line, they patched it and broke it. We are going on weeks without a fix.
In its current state, I can think of no reason to purchase this game unless you simply want a graphics demo.
1/10
Update: As of 3/2/14 the lag has improved from 30 seconds to 2-5 seconds of lag. It is still however, the worst I have ever seen and should be considered broken. Players warp around the map and die 5 seconds after you shoot them. It wasn't always like this. One of the patches broke it.
Buyer beware!