I... loved it, all the way through. I thought they did a wonderful job adding new game mechanics and the story kept my attention wonderfully. True, the cutscenes get a little long, but in the end it does provide a great resolution to the story.

Act 3, I believe, is the first act that really requires you sneak exclusively. You can get through the other parts with stealth, though you're also given the option to run-and-gun. I would play at least until you get to experience that. It was one of my favorite parts of the game. If you still aren't enjoying yourself at that point, then you may want to return it.