I played this game a few months ago having heard all the hype about it, and never finished it. I thought the same as you - it just gets boring. I kept hoping that the next level would reveal some kind of new game mechanic, level design or even just a new type of enemy, but I soon realised it was never going to. I think I started to give up after it railroaded you into the incredibly tired "defend this room for 5 minutes while hordes of enemies attack!" set-piece. I could see it coming a mile off, and could tell they were expecting me to find this exciting or something. What a shame, it was a cool idea.