Well, I love it ! The Smash Run mode is fun, especially when you have to reach the finish before your enemies or climb higher than them ! I also like the stages, the characters and the musics. It's really great !
On the other hand, I don't really like the online mode when you play with a lot of people for the moment. It could be fun, if there wasn't so much lag. (My 3DS even froze 5 times during a team battle in "For Fun" mode!)
Conclusion: It's an excellent game, if not a perfect one. However, wait a bit if you plan on smashing online. Too much lag kills the fun.