As the first game in the SSB series, this is game is pure greatness, just like the later ones in the series.
Single-player mode is fun, and, as the name would imply, is for one person only. It is rather different than the single-player mode SSBM and SSBB has: you fight the same opponents in the same order every time, no matter the difficulty or how many times you play it. This does dim it down a bit, but it keeps you set for who you are to fight next. The one-player mode does get a little more dull than the others in the series do, but it is still fun to play.
The multi-player mode is the kicker here: it's what the series is revolved around: knocking the snot out of your friends' players. With the players you can choose, the stages, and the items you can attack your enemies with: this builds hours upon hours of fun. The fact you and your friends can attack each other, or team-up and attack the other team/teams with numberous weapons; it never gets old. Even if you do not have any people to play with/against, like in my case, it is still fun to train and battle CPUs, but it's not as satisfying as attacking friends. Still, it is fun, even if you are just battling by yourself.
The unlockables are few when compared to the unlockables in the rest of the series, but rewarding. 4 characters to unlock, 1 stage, and the sound track, may not sound as big as what you could unlock today, but they definately add to the fun.
Overall, the graphics are good for the N64, the controls are smooth, the unlockables aren't many, but are still worth it, the appeal is great, the single-player mode gets a bit dull, but the kick-butt awesomeness of the multi-player mode makes up for it, and pretty much anything else you could think of that's wrong in the game(none that I can think of): this game deserves at least a 9.5!