If you are the host, everything is fine, but if you join someones game, after a few mins, you start to lag BAD! Blocks that you have already mined will reappear. Takes a few seconds to pick up mined blocks, even when your standing on top of them... You can hit someone in the game with you & the action wont happen for them for SEVERAL minutes later. MP is just completely useless & broken in this game.
Mojang, 4J, if by some weird chance your reading this, Please fix this.