you know what…. I really dont know what to say. I personally prefer a game that is good moment to moment over one that is meh moment to moment with a story payout later. So to me a game like heavy rain is not great at all. at least bioshock had more a balance of moment to moment goodness with an unfolding story. The story in portal 2 is interesting and there and the stuff you do along the way is fun. The story does unfold as you play it. I think you should play though portal 1 first because its very interconnected. I think if you finish portal 1 and love it by the end then part 2 is the game for you. I would be surprised if you played part1 in entirety and did not love it.
