Good, but nothing special.
The consensus is one of underwhelming. I didn't play with the fancy-pants features like PlayStation Move nor 3D blubbery, not that I'd ever want to - I doubt either feature would give me any sort of "enhanced gaming experience," as advertised, because when it comes down to it, the core gameplay and plot of the game should be the bit delivering an "enhanced gaming experience," not gimmicks.
Killzone 3 drops you in the middle of an ongoing conflict between the ISA and brutal Helghast regime. And it begins with a direct defiance of orders within the military ranks. Your hotheaded buddy-pal gets the bright idea to cap some high ranking Helghast official, and while this is apparently a very serious action to have taken - a very bad action, at that - he gets little more than a stern talking to and an angry finger wagged in his face.
This direct defiance of orders seems to be some sort of theme between the main duo involved in Killzone 3, leading to the worst organized military endeavour in the history of organized military. Your characters seem to do everything within their power to disobey their superiors, wandering around and causing unnecessary damage and risking the lives of tonnes of other soldiers. Granted, your characters are trying to do good - but after constantly spending the game neglecting their leaders, you'd wonder how they manage to get anything done. The plot carries out in a very fast paced manner, and while you don't need to be invested in the series to understand the goings-on, you might be left behind if you don't pay attention at least when the people talk. Fortunately, the plot is as thick as it needs to be, and this "without-crust" tale of gritty space-war adventures isn't too deep.
The graphics are simply stunning, that is to be true. Everything is so vivid and powerful. Even though half of the game is spent in blown out city-centres of the Helghast stronghold, there is a strong sense of immersion. The blizzard levels in particular make you feel wrapped up in a snowstorm, making the firefights ever more intense. Sound is also of high quality, but these days - especially for games such as this, with enormously large budgets - that's no flashy feat.
In the end, however, Killzone 3 isn't much to boast about. It's pretty, and the story is enough to perk an ear or two. But unless you're a huge fan of the Killzone series, you're not missing out on much. In the end, it's another tail of über-patriotic democracy-loving space marines trying to stomp out the threats of totalitarian dictatorships. Nothing you haven't seen before in dozens of other FPS games.