I downloaded Army of Two: The 40th Day a day or two and ago and I have to say, its not very different than the first one. The controls are not very intuitive and take long getting used to. The graphics are generally good, but not the best. I got bored about half way through the demo and quit. Here are few reviews from some gaming sites:
IGN(85/100): "A disaster movie in a game package, Army of Two: The 40th Day capitalizes on its co-op play to deliver an awesome action experience that's incredibly fun to play."
Official Xbox 360 Magazine(80/100): "Marry that to some satisfyingly chunky gunplay and the odd spectacular, building-leveling explosion and you have an action game that is far more fun than it has any right to be. You get plenty of co-op games, but not that many 'buddy' games and this is a fine example - a frantic, punchy bullet-storm that's best enjoyed with a friend."
1UP(B): "Even though I think they ultimately contributed to the Army of Two's inconsistent tone, which moves from "flippant" to "serious business about how messed up people can get," the morality moments are what I'll remember most about The 40th Day."
Game Informer(65/100): "With so many great co-op experiences available to gamers, it's tough to recommend The 40th Day. Lacking a cohesive story, solid controls, key multiplayer features, and polish, this sequel fails to close the gap of mediocrity running through its core game design."