The Fight pad Is a lot nicer feeling than using the Wii U gamepad/3DS, however it isn't as good as an original Gamecube controller, BUT it is a perfectly fine replacement to a Gamecube controller.
The triggers don't make a click sound, which isnt as satisfying feeling/a little off. I would definitely use a Gamecube controller over the fight pad in a tournament/extended gameplay, but as it is, I can easily use the fight pad and lose no loss to my Smash ability.
the C stick is the same as the control stick so it is different.
It depends on the Wii remote, so batteries are essential. BUT that does mean it is wireless, and functions as a pro controller. This means you can use it in Legacy games/Wii games. I also like to move my gamepad to my bed, away from my tv, and lay down and use that as the screen. Really nice on a sleepy day.
It also has a Yoshi theme, which was my reason for buying it anyway.
Id say its a pretty sufficient substitute for a Gamecube controller, until adapters come out. Even then, I will continue using the Fight pad for casual play.