It´s worth it. Excellent gameplay from start to finish. And it can be playable. Especially if you go with the "silent assassin" route the second time with no alarms, no killings and such on hard settings. That is pretty challenging.
I think it's a pretty good value in the $20 range. Like others have said, it's a pretty fun steath action game with no replayability. Flip down the nightvision and kick some butt.