Of course, it's a bit of an injustice to say that the game is that close to the original-- if not much of one. The game itself has a wildly unpredictable amount of secret levels and characters, the five ? mark boxes for characters and levels not remotely covering all the secrets. The primary characters and levels have been listed by another person, but there are still a couple more. "Flatland," a replica of an old plasma-screen portable called 'Game & Watch', is one of the most bizarre arenas to play in.
That having been said, the game isn't all TOO different. Classic mode gameplay is definitely more fun, the graphics are far better (of course), and there are secret characters including Ganondorf and Mr. Game and Watch (the latter and his level being worth the price of the game alone). Nevertheless, it's hard to avoid that 'been there, done that' feeling in some versus matches. Despite some obnoxious items such as the Party Ball (makes a loud trumpet noise and releases random prizes-- these 'prizes' are usually bombs when you're underneath the target) and of course Mr. Saturn, the gameplay and moves vary very little from the original. Thankfully, Nintendo's included a couple of interesting tweaks. Stamina mode, where you have HP rather than %, is short-lived but quite fun indeed. And, of course, you have slow motion mode-- quite handy, considering that the human eye is usually unable to see where the hell Fox is going. (If I'm the one playing him, the answer is "off the stage".)
The parts of SSB:M that appeal most to me are probably the new characters and new levels. Big Blue and Mute City are some of the most amusing levels to play in the game-- the latter is a moving platform that occasionally requires your getting hit by very fast cars, and Big Blue is a gigantic racetrack in which you have to balance on varying F-Zero cars while fighting. The characters have been improved upon to include various Fire Emblem characters, Young Link, Falco, the previously mentioned Mr. Game and Watch, Mewtwo, and so on. Samus herself has been slightly improved, now having faster movement and homing missiles. Having tournaments is a very fun thing when there're more than four people.
All of this being noted, in the end run it's an excellent idea to purchase the game. It may be essentially more of the same, but to use HL examples, it's more of an Opposing Force "more of the same" than a Blue Shift one. On its own, it's an excellent and enjoyable game, even for only one person, although that can stretch. It's not a totally different game, but it's been changed enough to be fully worth the purchase.