The demo is open to everyone now. I'm not playing the actual game demo due to any spoilers but the multiplayer aspect is cool. The concept is the better u do in the multiplayer, it helps your overall in game mission to beat the reapers in the regular game. You don't have to play multiplayer at all, but I think they did a great job with it so far.
Will it replace COD? no not at all, but I don't play COD