The first thing that I would have to say about this game is that I was drawn in by the characters. The story line, voice acting and general feel make it a good story, well told.
The playability and re-playability for the various outcomes is second to none. They have accomplished what so many other sandbox type (not that this is a sandbox game) games have not in that your choices really do have consequences. Even the route that you want your character to develop will make some obstacles very easy. I like the in game challenges and found them genuinely unique and at times challenging.
However, where this game misses is on some of its polish. The graphics seemed old. I grabbed this and played it as a used game and thought that it was one of the first to come out on PS3 and was shocked to find out it was a 2010 release.
The controls are difficult and awkward making game play difficult at times.
As for the various skills available, there is no real need to develop more than a few of them and as the other reviewer pointed out, there is no need to actually operate covertly because the pathetic AI leads to some fairly easy kills. I only had trouble with a couple of the bosses throughout the entire game.
In summary, worth the play because the story will suck you in but the difficult controls leave this not accessible to a novice gamer, which is unfortunate because the story is compelling enough to bring new gamers on board.