Been playing Brawl for 5 hours, but had to take a two hour break so my mother's boyfriend's band can practice in the bonus room.
I haven't looked at everything in the game, but there seem to be TONS of modes to keep you occupied.
Brawl is the best Super Smash Bros yet, in my opinion. The characters feel a lot heavier than in Melee, but you'll get used to it, so don't expect to be able to blaze through expert mode without any trouble. The overall feel of the physics is the same, besides a little difference.
The Subspace Emissary is a LOT better than Melee's adventure mode. I've been playing the SSE for about 3 hours or so, but it gets a little repetitive. The same enemies come at you, you watch some cutscenes, you go through some doors, you fight a boss or two, you clear the stage, you unlock a character, and you repeat. The main thing about the Subspace Emissary is unlocking things, and there are LOTS of things to unlock. I wouldn't buy the game for the Subspace Emissary, but it's a great thing to have. It should add on a few hours of gameplay.
The level editor is a pretty neat tool. Your level may not look like the most beautiful thing in the game, but you can pretty much create the levels you've always wanted to.
Wi-Fi is pretty basic. It's like having another friend, even if you can't make friends. Of course if you play with a random opponent, it's like having a friend that you can't communicate with whatsoever. With no leader boards, you're basically playing for pride. The thing about Wi-Fi is, you don't just have to do Brawls. You can play tons of mini games such as home run contest.
Overall, Brawl is simply amazing.