Although it can get frustrating at times, overall an awesome game.
Really, it's a fighting game that's unlike any other - you don't beat on each other until one dies from a health bar reaching zero; you beat them until they go flying off the screen in any given direction. It has a simplicity to it as well, only using two attack buttons, movement, and a guard button.
Now, I'd like to say now that it isn't without faults. The single player, though it has had drastic improvements, has a few things that make me want to slap the producers. Namely, the inability to switch between characters in missions (save by suicide) and the sometimes frustrating platforming levels. The platforming itself isn't that bad or tedious; it's the fact that if you miss a jump you can reappear with your new character hovering over a cliff and lose two lives for the price of one. That aside, the single player's silent storyline is certainly a change from "Beat stages until you meet up with Master Hand".
Naturally, now is when I move onto multiplayer. Brawling with your friends on the same machine is as fun as ever. However, for those of us who don't have a T1 internet connection hooked directly into our Wiis, brawling "with anyone" online can be rather laggy and you're limited to a standard 2-minute brawl. Brawling with friends can still be a good experience online, however, if all combatants happen to be near; in other words, get your friends to pick up a copy and fight them. It'll be much easier.
Finally, the difficulty - Versatile modes from easy to intense cater to a wide variety of skill levels. For those of you who like to explore, there's plenty of secrets hidden in the levels, so finding that last hidden room might add in some fun. Those who suck at finding secrets, well...sorry. You might not do so great without a guide.
Overall, this game is a solid addition to the series. Though I didn't mention everything (mini-games, which are few), I believe I've covered the main parts of the game enough to help you make a decision; not that I believe many will choose not to buy it.