Super Smash Bros is absolutely mental. It's pick up and play, the gameplay is extremely fast paced and frantic, but most of all it is fun. You can fight as traditional Nintendo characters such as Mario, Donkey Kong and Wario (who has a pretty amazing motorbike that he uses as a weapon) but also with the likes of Pikachu, Villager (from Animal Crossing), Wii Fit Trainer (!) and Pacman (my personal favourite).
The game has a few different game modes such as classic brawl, and also a series of set challenges to complete that earn you gold and trophies to add to your in-game collection. You are encouraged to play the game through with each fighter, which takes some time. Fantastic and well worth the money, even if you're new to the series...I'd played Melee and Brawl previously and therefore knew what to expect.