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..This is one of my favorite games, simply because of the stealth.
After each round you get graded for how well you made it through undetected. 100% is great, but when I play I try to go through the whole thing without leaving any trace that I was even there. It's really satisfying to think about how all of those guys I sneaked passed on the bank job are going to get fired the next day for incompetency.

I didn't really like the multi-player, but the co-op is frickin awesome. playing a stealth game solo is fun; having a stealth teammate adds a whole new dimension to the game. It's a real test of self-control, too, when your ninja skills give you the upper-hand. You know that you will lose points for killing an enemy, but it is so hard not shooting an enemy in the face when your friend has him in a choke-hold from behind.
Whenever Doc Brown and I played the co-op, we would split up to complete separate objectives, and reconvene where teamwork was needed. It's hilarious to hear gunfire happening in another location, alarms going off, and then you get the panicked "I need some help!" call over the headset. You then have to try to maneuver to your friend's location while staying as covert as possible.
The AI isn't extraordinary, but they say some really funny things, and they become more prepared & aware as alarms are triggered. Also... your mics are sound-sensitive in game, so if you talk too loudly to your teammate the enemies can hear you.
Great game. and the graphics are still pretty nice (except for the skyboxes)
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