So I picked this up on that Amazon sale not long ago and finally got my ass to the book store so I could download it. So far, its pretty great. And I haven't unlocked anything or even played the second level.
Theres probably some crazy replay value in here. So I went through the first level like a n00b, got seen a couple times, fucked up my stealth kills, etc. So instead of moving on to level 2...lets do level 1 again! This time I made sure to nail my stealth kills. Then I find you get bonus points for letting guards pass you undetected, and then you stealth kill them, then you hide the body for a whole bunch of points. It also seems pretty lenient in some ways. Did you just screw up a kill? Reload checkpoint and do it again. Thats very useful because there are these secondary objectives in the levels, like "Rescue so and so without being detected." I was going for that and got detected a couple times, but reloading the checkpoint also reset the status of those objectives so I was still able to earn it.
Anyway, its quite fun if you like being all stealthy and murdery.