Now I should get this out of the way, I don't hate miis. They are the decent enough.
This has up sides and down sides to me.
For the good stuff, miis can offer a lot more variety especially given their differing appearances and move-sets.
On the other-hand, I do look forward to to what original stuff they bring to the game. I mean, who can forget about sand-bag? The polygons, wireframes, and alloys were all interesting in their own way and I was looking forward to what big thing they had for the next multi-man. While miis are technically smash original due to the context, I just doesn't feel the same.
Again, this is a good idea, I'm ok enough with it and it will be a fun game-mode.