I have been a big fan of the Smash Bros series since the N64 days, and Brawl is the best incarnation yet. The customization features and balance really raise this title above Melee, which was already a fine game.
I find the single player campaign to be a bit of a let-down, but nothing compares to the real heart of the game: the local multiplayer. Though not quite accessible to the casual set, Brawl is easy to pick up and play for those familiar with fighter/platformer gameplay.
A half-point must be deducted for dismal online play. No voice chat, horrible lag 75% of the time, and a feeling of isolation and lack of customization make the online multiplayer a real drag. If you have a number of friend-codes plugged in, a reasonable game can be set up, but Brawl is truly intended for local play.
If you are a Nintendo fan, you simply must buy this game. Heck, buy it if you own a Wii. Just beware of the problems with online multiplayer.