I would like to know what are the options when you grapple an enemy like can you shoot whit him as a human shield, beat him up, watch him agonize after diving your knife in his neck and can you hold him for ever or will he be choked to death and if so, how much time can you keep him

Also, you know when you land from a big fall and there's an animation where your hand touch the ground, I wonder if you grapple a guy and jump of the roof and land far below, will you crush the guy in the ground like in the trailer? skip to 2:47 http://www.gametrailers.com/video/dead-men-crysis-2/710561

IT WOULD BE AWESOME!!!!

You use up energy while you've got the enemy grabbed so it's not infinite. All you can do with the person is throw him away from you just like you can by grabbing other items as well.

@grapplejumpoffbuilding: It's probably going to be a scripted scene in the campaign, IDK, I haven't played the game much.

but I saw a video where you could do multiple things when you grabbed someone...WTF!? Could someone look in the manual or something?

yup all you can do is throw him. but it works well when you power up the throw. Throwing enemies into enemies is always funny

You can knife/snap neck too, its called a melee button, use it.
Basically making this a second chance for a failed assassination.