Simply put, if you have a gamecube, you must own this game.
Gameplay/Story: not too much in the way of a storyline but thats fine. It really doesn't need one since it would probably end being really little kiddish and detract from the game.
The game's greatness is really taken from the simplicity of the gameplay. It really doesn't try to make an in-depth fighting system and it shows. However, it's made so perfectly that this lack of depth means that newbies can play as well as experts. And it isn't the kind of game based on luck where the newbies can still beat the experts just from getting lucky. It's possible but highly unlikely. There is so much strategy and reflexive skill that takes place in SSBM that if you are good, it will show. The game is really about predicting your opponents moves. Knowing when they will attack and if they do attack, if you can dodge or block and which will work better. If you block, they will probably try and throw so can I get away from their throw after this attack is done? The person with the most skill is the one that can predict his opponents moves and that's where the greatness of SSBM lies. It's perfected this so that newbies can pick up the game and play friendly games with others and experts can have epic battles among each other as well. Finally, if you want to play with experts and newbies in the same battle, they created a great handicap system which allows the newbie to fight at the same level as experts. This is completely optional though so you don't have to use it if you don't want to.
The game has 32 characters to choose from total and each comes with their own special B attacks and own stats. These moves and stats are usually derived from how the character fights in his/her original game. I.E. Link will use his boomerang and bombs while Samus can use her charge up gun. Also characters like Kirby or Jigglypuff will go flying farther from getting hit whereas a character like Donkey Kong won't go as far but is slower. The great part of the characters is that there isn't a single character that is bad in the game. Many people will say "that's not true! Jigglypuff and Zelda both suck!" I have multiple friends that are incredible with both characters (and that's without using jiggly's sleep. With it, they're almost unbeatable lol). Say you really like pokemon so you want to play as pikachu. no problem, you don't have to worry that your favorite character, pikachu might not be able to stand up against someone like Link. All the characters are perfectly balanced.
Also, there are tons of different courses to choose from for every type of gamer. There are still, small courses and there are huge, constantly moving courses and everything in between. Something for everyone.
Finally, the game options. You can honestly customize everything about a battle. You can turn individual items on or off, change how often items come into the battle, turn handicap on/off/auto, change who picks the courses, damage ratio, literally anything you want you can change. This also means that if you want to play a quick match with a couple friends perfect, or if you want to play a long one on one bout with someone, that's fine too.
Graphics: The graphics are quite nice for a launch title on the GC. All your characters are depicted nicely and are true to their characters. Pikachu looks like pika, Link looks like Link should. everyone is great. And the moves are all done perfectly. no clipping errors or anything like that. You'll never get the feeling of "WHAT that did not hit me!!" If an attack hit you, you can see it. Not to mention all the backgrounds are nothing less than eye candy.
Sound: The music is a great collection of remix's from your favorite games. Play in the dreamland course and you get a remix of the classic Kirby theme song, play in Big Blue and you get some F-Zero race music. Theres plenty of music that all go perfectly with the game. Also, the characters make realistic sounding sounds that truly fit when they get hurt or attack. But these sounds aren't so frequent as to annoy you.
Value: You will most likely never get tired of this game. I've been playing it ever since it launched and i still play it just as much as i did when i first got it. Theres so many single player trophies and other things to unlock that even when your friends leave, you'll still have plenty of things to do in the game. Of course the game is best played with more than one person. Especially when you get 4 people match's going, you won't ever want to turn it off.