Great!
There is some superb satisfaction from sneaking past guards, nearly right under their noses, killing their boss (or VIP) and bailing before they even turn around.
A difficult game made all the more fun by the randomised paths of the guards and the targets. Different every time, requiring (as you mentioned) lightning fast reflexes to avoid being sighted. It was interesting that an assassin would go in without killing any of the guards and using only a small knife, but I suppose that is what made the game so uniquely challenging.
One problem I did have was that the guards sometimes saw me even when I wasn't in their cone of vision. If I stepped out next to them when they'd walk past they'd turn and shoot me, even though they could only see straight ahead. Slightly frustrating, but a little more realistic.
The trail of bloody footprints initially worried the hell out of me, but the guards more or less seemed to ignore it and run around frantically. It was a cool idea but didn't seem to have any effect.
Also, it was supposed to get increasingly harder, but I found some of the earlier stages to be infuriatingly difficult. More guards and fewer entrances do not necessarily make a stage more difficult if there is more cover to take.
If I might make two recommendations, just for kicks, I'd give the option to use a distraction of some sort. To throw an item that makes noise, or to leave behind a small grenade or something to draw the guards' attention. Secondly I would recommend that the target react if the alarm is set off by a guard sighting you- security around him/her might increase for awhile and they might stick to one area rather than wandering about the stage. It would make the game a lot more difficult, but that slight bit more realistic.
Overall, a surprisingly satisfying stealth game, though some sort of reward (unlockable content perhaps?) would have been appreciated for becoming the Ultimate Assassin after trawling through 18 levels.