The game is more fun if you play it with friends. While it can be fun to play for awhile on your own, unlocking all the challengers and trophies, the real fun of this game comes from online battles. Again, while it can be fun to play against strangers, it's friends that really bring this game to life.
If you've played any of the older Smash games (I've played Brawl, released for the Wii) you will notice that some characters are no longer in the game, such as Solid Snake from the Metal Gear Solid series. However, new DLC has included Corrin (Fire Emblem: Fates), Cloud (Final Fantasy VII) and Bayonetta (Bayonetta) amongst others, which can fill in that gap. There are also new challengers already in the game, including Robin (Fire Emblem: Awakening), Charizard (Pokémon) and Villager (Animal Crossing: New Leaf) - again, amongst others.