I played the original Super Mario Bros growing up and I can't begin to describe how much fun this game is. All of your favorite enemies are back, looking better than ever, with new tricks up their sleeves in their attempt to stop Mario from rescuing Peach.
The levels become more challenging as you advance through the worlds. Let's just say that it's a good thing the Wiimotes have a protective coating haha.
Co-Op (esp with 4 players) usually degrades into chaos. So as long as you don't expect to get anything accomplished, you'll have fun.
I bought my Wii specifically for this game and was not disappointed. You won't be either.