Karen and I bought the other Super Mario game and were really disappointed. It was a good enough game and really what most gamers are asking for in a return to retro. But maybe I've lost a step, or a finger, cause the game is just too damn hard! I think we died twice on level one alone! It only got worse from there. Every level was a bitter struggle. We must have continued 30 times. I think part of the problem is that two player sort of forces you to go slow, and that really makes things difficult. You need speed and a flow to really knock out out those difficult sections, but if one player gets too far ahead, the screen stretches out and eventually the other player gets killed. There is an ability to "bubble" allowing the second player to safely float back up to player one until they are popped, but it just makes things more complicated and again, slows down the pace.
Super Mario Galaxy on the other hand, is a masterpiece. The difficulty is just about perfect for Karen and I. She can handle most of the levels just fine, but if anything particularly difficult comes up its easy to switch controllers. The second player is involved, but not absolutely necessary and they don't really ever get in the way.