Overall, Army of Two The 40th Day is a much more polished experience than the first game and greater care was exercised in trying to address the concerns everyone had from the original title. Playing this game with a friend is still the best way to do it, however, with the A.I. improvements you will be fine going through this game solo as well. Multiplayer can be a lot of fun when it is working correctly and players should have fun with the level of customization that EA has put into this game.
The morality system works pretty well and does have some consequence too it as you make those hard decisions. With all of the games coming out during the early part of 2010 be sure this is on your list of games to try.