A great singleplayer mode will keep you playing for weeks, but the multiplayer is where the fun really begins.
The singleplayer portion of WarioWare: Smooth Moves is pretty fun. There's no real large storyline here, but each character has a mini-story that sets the mood for their microgames. In case you don't know, microgames are a new type of game that WarioWare invented itself, and they usually last about 5 seconds. They'll make you do stupid, wacky, zany, and silly things, such as pick someone's nose, close up a shop, or even shave off a poodle's fur. While the singleplayer mode here is certainly very fun, it doesn't last quite long enough, and it will only take you a few hours to go through each character's set of games. That said, though, you will want to keep replaying all the microgame sets, not only because they're fun to try and get a high score on, but also becuase some fun rewards are unlocked upon playing every single microgame hidden inside WarioWare.
The multiplayer is just hilarious. It mostly consists of either a new game for use only in multiplayer, an expansion on an existing microgame to form a long-lasting and enjoyable multiplayer experience, ot a collection of microgames structured in such a way so that they can make anybody start grabbing for the remote in WarioWare's zany multiplayer. Some modes are disappointing (balloon), but most of them are pretty darn fun. I think my two favourites are Darts and Lifeline. Bungie BUddies is also fun, but it isn;t quite as accesible as the other games because it only supports two players. it would have been neat if you could use multiple controllers to control 3 or 4 seperates teams of buddies. But these are minor errors, and the multiplayer here is one of the strongest parts of the package, and it's the main reason that you'll wanna buy WarioWare: Smoth Moves, not rent it.