The single player game isn't terribly great but it's at the very least competent and suitably action packed.
The multiplayer game has features that I take serious issue with. Notably, intrinsic imbalances in maps, game mechanics that favor one side over the other for given missions, a class system that seems fundamentally broken and a world that is always either too large or too small for the group you are playing with. The general result is that the game fostered a style of play that I just didnt' find terribly compelling.
For example, when I say the class system is broken I mean exactly that. While the game sports several "classes" you quickly find out one thing - your starter class is almost entirely useless. While the default "soldier" class can wield almost any weapon in the game, you are not alloted any additional abilities or skills. As such, if you wish to use a rifle, there is no logical reason to not be a Tactician or Medic instead as both wield an automatic rifle and have a valuable support skill to boot. If you want to use a shotgun, then there is no reason not to be an engineer as this class can build sentry guns. What's worse, you'll find a host of quiestionable class balance choices throughout. The assault class carries heavier armor (thus equating to more health) than any other class, all while having the enviable abilty to fire at the run or go for a full sprint at nearly twice the speed of other classes, all while being armed with either a rocket launcher (which is fantastically letal overa a wide radius) or a semi-automatic grenade launcher. The sniper class on the other hand gets a rifle that kills in a single shot (and not just a headshot) and has the capacity to turn invisible. Not only are they supremely lethal at range, their invisibility effectively shields them from the only real retribution they might face - other snipers.
Mission balance issues were another problem of mine. While I really liked the idea behind them, at the end of the day victory or defeat in a match often came down to how the missions were split up between the two teams. Assassination missions favored the attackers, defense missions favored the defender and so forth. Thanks to the non-linear nature of most of the maps, even the king of the hill archetype generally favored one side over the other, by placing two of the three points notably closer to one base than the other.
And map size was a constant problem. While there exists a map suitable for any number of players you'll likely see, I often found myself playing on a map designed for 20+ plaeyrs with 5 - 10, or worse still a map best suited for a handful with a full complement of players. The latter was almost certainly the most irritating as the matches often degenerate into bloody, brutal stalemates where no amount of skill seem to sway a battle or affect your odds of survival.
The part that ultimately broke the game for me though was how this meant people would choose to play the game. Often, if one wanted to ensure victory they simply needed to have a tactician throw a smoke grenade (that acts as a spawn point) very near or even inside the enemy base. In the ensuing carnage, the entire enemy team would be tied up simply trying to get out of their base while a handful of your own team members were free to accomplish the game's objectives.
That said, I havn't played the game since shortly after launch, and all of my qualms may have been fixed somewhere along the line in a patch. If they ever resolved the "you can throw a spawn grenade inside the enemy base" bit, that may well prove enough to bring me back to the game for a time. Of course, if they have not, I would say that the game is not worth the investment. At a fundamental level Killzone 2 players like a slower version of COD 4, so unless one has already played COD 4 to the limit of it's enjoyment potential I'd say that would be a safer bet.
