For those unfamiliar with Smash, it is a fighting game that throws together characters from different Nintendo universes, as well as a few from other companies' franchises such as SEGA's Sonic. Smash also incorporates a great deal of items, including the Smash Ball, that can change the entire pace of the game. Items flying everywhere and gravity-defying physics make for a certain degree of hilarity.
The sheer chaos that often ensues during a 4-player stock fight (that does not even consider how chaotic it could be with 8 players) is not only amusing but also allows for replayability, as you are rarely going to have the same experience twice.
This new version adds a few new characters, stages, and items, as well as cartridge-specific trophies. There are also a few DLC characters, as well as character customization. Super Smash Bros. for Nintendo 3DS also introduces Smash Run, in which you travel a maze and collect power-ups for a free for all battle ahead.
Overall, it is something worth having around even if you are just playing solo against the game itself.