As a long time fan of the MGS series, it kills me to say "Don't buy it."
I've personally played this game from start to finish and I despised it, I even typed up a review upon it. (As you do.)
I found nothing I enjoyed in the game, but then again, the things I disliked (everything), might be what people enjoyed, really, I timed, on a stop watch, from the second I pressed "New Game" until my first encounter...23 god forsaken minutes, and that's the problem with the game, it's 95% cut scene, 5% actual game play, it literally plays like a movie, with very little emphasis on the action.
Half the cut scenes go into detail more about things anyone with a brain could've picked up by paying vague attention to the first one, it tries to sell itself as a tactical espionage action, but that goes out of the window when you discover you have a gun that knocks everyone out and never runs out of ammo.
If I had to sum up MGS4 in one word, I'd tell you to stop being such a moron, but if pressed I'd go for inconsistent, (Or schizophrenic.) it can't decide what it wants to be, a movie or a game, there's painfully little action in the game and even less sneaking, it basically becomes little more than a game of Pac man with better visuals and guns.
I'll state one last problem, Snake may be able to climb over certain things, but there are others he just can't, if he can climb over a knee high wall, then he shouldn't have much more difficulty climbing over a chest high wall, or for that matter, blowing up a barbed wire wall, when things like this occur, that's it, the fourth wall gets obliterated in a shameful fashion.
Sure, this may have been lengthy to read, but I'm a critic, therefore I criticise, it's sad to think that after Hideo Kojima made so many cracking games, it was all let down by this one, I hope I helped in some way or form.