New Super Mario Bros Wii is a delightful conundrum. Some love it, others less so. The most significant addition to this iteration of the Mario series is the addition of 4-player co-op. The dynamics of this new co-op differ slightly, but importantly, from similar co-op titles. I think these dynamics are responsible for the wide range of reactions.
Unlike other 4-player co-op games, NSMBW isn't about everyone working together to succeed together. Left 4 Dead has "live together or die alone" in its marrow. But the collaborative goal in NSMBW is subtly different. The goal is really about working together to make sure at least one player succeeds. More often than not, the other players end up as kingmakers so one can succeed on everyone's behalf. In L4D and Rock Band, doing your personal best nearly always contributes to the group's success. In NSMBW, doing your best often comes second to taking one for the team.
Only one player needs to go through a pipe or a door, enemies don't become any stronger or more numerous and as long as one player is alive, the game continues and the players can still succeed (a feat impossible in all but the last few seconds of a Left 4 Dead scenario). The fact that players collide instead of sharing space a la Little Big Planet has been much maligned. And while frustrating, without this mechanic, players would not be able to bounce off each other to reach greater heights.
NSMBW has been a roaring success, to the magnitude of Rock Band, with myself, my wife and another couple. Our old school Mario backgrounds vary a bit, but it's been a blast for everyone. It took a bit to get past the initial frustration, but once we did we realized that it's really about us succeeding as a unit rather that succeeding as individuals. A favourite moment when when I held a portable spring with another player bouncing on top, synchronizing our jumps to reach absurd heights.
I imagine the dynamics would be more frustrating with just two players as well. If I was constantly killing my wife by accident or vice versa, I imagine it would be far more frustrating than the weal and woe being spread amongst three others. It's definitely a different dynamic and takes a while to adjust to, and could well not be for everyone. But for us, it's never been more satisfying to take a bullet.