This game is fantastic. It takes the best ideas from FC1 & FC2 and molds them into something more satisfying than the sum of its parts.
I haven't tried multiplayer yet so my experience refers to the single player campaign.
The first thing which hits you is how cinematic the start is. You're immersed in a Blockbuster from the word go. The voice-acting and the emotion conveyed by the virtual actors is first-rate - Vaas is an unhinged psycho who will shoot you in the face on a whim - the others are as believable as any I've seen.
Once the initial tutorial sections are over with, you're released in a free-roam, GORGEOUS world when you're as likely to die from a bullet in the face as the horns/claws/teeth of some irate jungle-island fauna - and that goes for the enemy! Several times I've been stealthing up to some fools when they've started shooting at the Komodo dragons I didn't notice slinking around in the grass adjacent to me. This world is alive; tigers sprint through a jungle clearing after Tapir , Leopards tackle deers on the beach, bull sharks melt into the shallows menacingly.
I could literally play FC3 for a month without touching the main story-line there's that much to do.
Each animal you kill, whether by bullet, knife or - my favourite - the bow and arrow - can be skinned. You'll need different skins to make different things - a larger loot sack, a larger ammo belt - a larger arrow bag. Plants too can be collected to make syringes which heal you or heighten your senses.
the adventure starts as soon as you leave one of the safe villages, you'll find animals, enemies to fight and things to climb for the hell of it or to unlock areas of the map. It's a very rich experience and a deeply satisfying one; take aim with your bow, which you've modified with a holographic site (yes, all the weapons can be modified with attachments), shoot and skin some animal you've stalked through the pleasingly rendered jungle foliage, collecting your arrow from its warm corpse and craft something from jungle loot!
The AI behaves predictably - this isn't a criticism. Throw a stone to attract a guard and he'll cautiously walk over to investigate the source of the noise, allowing you to divide and conquer with your trusty machete. Stealth is done very well in FC3 - if that's your thing - there is of course the louder option which plays out very well too - the guns sound and feel satisfyingly punchy with varying handling.
Graphically the game looks stunning. The character animation for bipeds and quadrupeds alike is natural - furthering the sense of immersion.
I turned off the music for no other reason than I always do, but everything sounds like it should.
I would recommend playing on the medium difficulty based on my experience - the island should be somewhat intimidating, at least to begin. You'll probably die a few times, but the game won't punish you unfairly and you won't have to re-do sections of the game over and over. You CAN change the difficulty on the fly if things are testing you too much.
The only negative I can think of relates to some key-mapping issues which may be my fault as much as the games.
Beware of over-pricing of this title - I see that console gamers and being asked £60-80 for the Insane Edition. I paid under £30 from Amazon so bare that in mind.
Over-all I can't recommend this enough - it's definitely the game that I'll be playing through into 2013.