The buttons fit, work and look great couldn't be happier with them.
A/B/X/Y L/R and C-Stick fit perfectly in my OEM smash controller, but the z button and left stick did not fit on it at all
All of the buttons fit within my original controller with minimal wiggle, no more than the old ones.
The thumbsticks have rubberized caps, just like the original, and fit snugly on the sticks without being hard to put on.
The only button that didn't fit was the Z-button, but I'm sure with some minor trimming it would work