I agree that not many top down shooters were good (zombies ate my neighbors is the last one i can think of that was good)...however the first Assault Heroes was awesome with a friend. And this one looks like that formula but more polished.
Metacritic says an average of 81 so it can't be too bad. :)
http://www.metacritic.com/games/platforms/xbox360/assaultheroes2