This has to be one of the best open world FPS games ever made (GTA, Just Cause etc are TPS).
The environment that they have created is exceptional - the flora and fauna really turn it into an engaging and believable tropical island. Considering the amount of vegetation being drawn at any one moment, the draw distance and frame rate are superb for a PS3 game. I can't think of an openworld game on PS3 that runs better, considering how much is on screen.
The AI of both the pirates and the animals is also very good, clearing a pirate outpost with stealth alone is a real challenge. Completing everything the game has to offer could easily swallow up 100 hours of your time. Collecting memory cards, relics, WW2 letters, Rakyat missions, vehicle time trials, bird shooting, poker, knife throwing, main missions, radio towers etc - the amount of content is huge. The RPG elements such as building your skills via a tatoo and creafting syringes (powerups) and bags (ammo and loot) really gives an FPS a bit of depth. The main missions, 38 in total offer up some interesting story and character elements and also unlock your skills - although they are a little bit too drug induced for my liking. You'll also love the first time you get pounced on by a tiger, shark or croc - I nearly jumped out of my skin !!
It's not perfect - I would have liked to have seen a greater variety of vehicles, maybe planes, tanks and helicopters to add to the hang gliders. Also, a return of the mortar from AC2 - or even air strikes or artillery would have added a bit to the game.
Overall though, it's easily one of the best openworld games out there. As for the comments below - the only time that you are restricted or on rails is during a main mission - the rest of the time you are free to roam and hunt as you please.