Count me in that percentage that hasn't failed.
As much as I like, nay, love the game....I think one of the issues that has led to my non-completion is that simple mistakes based on vague information (who really is the target, when does the mission actually start/fail, what patterns are A.I. making) makes you fail some of the missions, and because the 'game save' mode is a little quirky, often you must repeat the entire sequence and sometimes those can become annoyingly long.
For example, early on in the game when you have to whack the guy who is talking to some schmoe on the above groudn subway platform and he runs down the steps.....Well, I failed that a few times.
The painful part? The 10 minute+ drive all over again across the bridge, pay the toll, navigate the streets, park the car, walk up the stairs, cut-scene, then chase. Yeah I could do without having to drive over and over again.
Thre are a few other examples of this that can take a failed event and turn into a 10-15 minute repeat action without a more forgiving save component that kinda makes it frustration for progression. So maybe I'm a poor gamer and my reactions aren't "all that" anymore, but after failing that mission the first 4 times, I realized I spent over an hour just starting all over again.....not fun.
