The campaign has the feel of some classic Mario games (the Koopa Clown Car alone is worth downloading the free first three levels of the game), and is a lot of fun, if incredibly short -- you can play through the entire campaign in about an hour.
Toad Rally really helps to stretch the game out. Toad Rally helps you expand your Mushroom Kingdom, unlock buildings and decorations, and forces you to improve at the game, otherwise you won't be able to compete for very long. It's a unique addition to the game that, while I've never gone into a Mario game looking for online multiplayer, has certainly grabbed my attention -- as of December 31st, I've probably put in an hour or two every day just in Toad Rally.
The game requires registration for online play, a feature that, while standard, seems absolutely ridiculous. I wonder why I accept this invasion of my privacy; Nintendo doesn't need my email address to know that I suck at video games.