Runehamster: Is it linear mission based, loose mission based, or random freeform run and gun? And do enemies respawn, or is the goal to 'clean out' various areas?
stonebro: It has a bit of both, although for a supposedly complete "free-roaming" game there's a high corridor shooter factor later on.
Some levels are like the first though, with basically a whole island, or sometimes several, to just explore and find the best way to complete the objectives on. Charging head first into battle never is.
About the "free-roaming" thing, if you travel too far you "summon" a killer helicopter. Of course, "too far" is relative. You can go anywhere you can see, like to all the little islands that are just put there to "fill out" the archipelago the game takes place in, but the "free-roaming" thing is meant for finding alternate paths to your objective. Straying too far away from the general area of your objective, like to one of the furthest islands when your objective is deep in the jungle, will summon the chopper.
Of course, if you ever get anywhere near "too far out", the guy on your communicator will warn you. Once.