Good but extremely over-hyped. It is NOT a classic, but you will have fun!
I have read so many nintendo freaks go NUTS over the adventure mode. And good lord, I just completed the adventure mode today and it was a BORE!!! All it is was the same side-scrolling adventure with the occasional boss fight. The "storyline" was very vague and boring, and did not capture my interest whatsoever. The movies in between bosses and stages are nothing special. I just beat the adventure mode today and felt really unaccomplished after it was all said and done. I think it could have been way better, and thought it was going to be with all the hype, but I was disappointed. It sucked.
I think my favorite part of the game is simply the roster. There is a very good mix of different type of characters so that one character doesn't feel too much like another. The Final Smashes are nice, but some are incredibly over-powered whereas some are just kind of, meh.
I had to ask myself, "Is this really THAT much better than Melee?"
The answer is no, not really. The gameplay is absolutely identical and in no way different from Melee. Let me say this, if this was the first game in the series, it could probably be labeled as a classic. But Brawl does not give you the feeling like you are playing a brand new game. Really, it feels like an expansion to Melee or the original.
In conclusion, if you don't enjoy playing this game, then you don't appreciate fun. Multi-player is fantastic (with the exception of WIFI) and can go on for a long time. But do not put this game and classic in the same sentence! It is not even close to being a classic and is incredibly over-hyped.
Here is a list of classics. Super Mario Bros 1 & 3, Tetris, Mario 64, Zelda-Link to the Past, Zelda-Ocarina of time, Metal Gear Solid, and FF7 to name a few. Those games are REVOLUTIONARY! Brawl is just a slightly improved Melee. I assume 9 out of 10 people will enjoy this game, but rent/play it first before forking out the cash.