Played through it... awesome. I used to fantasize about crossover characters when I was a kid.
It's also a good starting point for discussion about classic 2D game design. Clearly the "guest" characters don't have the optimal abilities for dealing with the world of SMB, and it goes to show how well-designed Mario's moves and powerups were, and/or that the level design suits the character's abilities. Indie developers should take note when doing balancing and playtesting.
Of course, sometimes it's just fun to turn off the brain and use Bill Rizer's machine gun to blast Bowser in the face.