Not being sarcastic it really is the game that the first game should of been to begin with. It has 12 new characters and a spectator mode in it. If you haven't the first game yet I would hold out for this one if you are interested.
There is one catch though, even if you bought the first one and want to play competitively online or offline you are going to pretty much be forced to the new game. The competitive crowd will move to it. There are some tweaks and balancing that will happen in the new game along with the other characters that make it the tournament game.
So if you want to play alone MvC3 is fine, if you want to be competitive with it you must go to UMvC3.