After playing it at the local mom and pop shop for a long while. 360 and PS3 version btw. The game works best when you are doing stealth and playing that way. Soon as you get caught which still happens with enemies seeing you through walls etc, the game gets incredibly choppy when there's any kind of decent sized firefight unfortunately.
Bunch of weird issues with this game. For instance, when you go to perform a stealth takedown, outside the first couple levels where they are holding your hand, crouching and slowly advancing almost never works ever. Plenty of times where I'm creeping up on a guy cloaked, crouching nice and slow and then he just turns around and starts firing. Best thing to do is just walk right up to them and stealth kill them. The other big issue with the game is enemy inconsistency. Half the time, enemies are finite, as in, even with the backup coming from alarms you can kill everybody and move on. Some stages though, enemies will spawn non-stop until you move to a certain point. Worst part is it never makes sense as to why some places are like that or not.
I really dig the game, has cool set pieces and amazing music, but the wonky stealth mixed with tons of enemies coming in which in turn destroy the frame-rate take me out of the game big time. Sorry for any typos typing this from a phone.