Adding myself to the list. I was able to connect ONCE to a co-op game (to a guy on my friends list). We played the first level about half way through, then crashed. After that, we couldn't connect again, and we tried at least 10 times before giving up. Like the rest of you, were were stuck on the forever loading screen.
I bought this for the co-op experience. I have no interest in playing through it in single player mode. If MS doesn't fix this problem within a few days, I'm getting a refund (I don't care if it says "there are no refunds for this item" in the marketplace).