The SSE mode is basically a fancy, new, 3D side scrolling Mario game with different characters, and an occasional cut scene and boss fight. It does get a little repetitive after a while, but I did find it fun to play for the most part, especially with a 2nd player. I'm actually quite happy they allow you to have a 2nd player.
And I don't really get the story line either or what Tabuu has anything do with it. The enemy is obvious, but I don't really see a reason why they do it. They just do.