I'm about 60% of the way through, for the most part I'm enjoying it but the missions do get a little irritating, there doesn't seem to be too much variation and I've found it quite irritating that you can effectively fail a mission and still continue through the story. It's still very good but I can't see me playing this again after I've finished it, much like Portal, I feel it's an experience rather than a game in the traditional sense.