This is without question one of the greatest games I have ever played. The sheer amount of gameplay options is staggering. The places you visit, the things you do. I am in a random base in the middle of the game and that location could be the final level of any super well designed action game and here it's just what you do for one mission, no big deal. There are some games where the stealth and action is broken up in tiers like Deus ex, you can build the stealth tree or the combat tree, screw that, here you are everything at once. You are not half a soilder. You have every bit of stealth mastery that you have for combat mastery and it all happens on the fly.
I have gone 30 minutes of perfect stealth, systematically taking out guards making my way in and one mistake, boom now I am fighting guys with shields, snipers, a helicopter is firing missles at me. It's ridiculous and that's just normal. I can't tell you he thrill of evading the alert phase when entire death squads are searching for you and you manage to make the perfect distraction and slip right by them.
I can't think of a game that has more satisfying emergent gameplay than this. Not GTA, not far cry, this sets a new standard.