So, I got the Wii Fight Pad. Here are my thoughts on it after playing Project M and Mario Kart 8 with it.
First things first, though it may be bulkier, space is a slight problem here. My hands are slightly above average size, and while they grip nicely with my pinky and ring fingers it took awhile for my middle fingers to adjust to the back that's thick, whereas the original gamecube controller has a slimmer back thus having more room. The weight is lighter than a regular Gamecube controller too, probably due to no rumble though.
The analog sticks are similar to those of the classic controller pro's but bigger. I used my classic controller pro for Project M, so this isn't an issue for me (I actually like them more than a regular gamecube controller!) However, the first analog stick sticks out more than the right analog stick, so this may seem awkward to some people. I adjusted and actually like it being pushed out farther than the right analog stick. This is may be just me, but I feel like I have more control with it. Also, having an actual analog stick over a C-Stick isn't as bad as you think
The R and L buttons give a very satisfying click. ZL and ZR are similar to those of an actually gamecube controller without the full press click. The shape of the triggers are different too but also have less space for your index fingers compared to a gamecube controller. This won't be the same with everyone, but sometimes my fingers overlapped with the plastic when doing a fullpress. However, it isn't deal breaking and simply moving your fingers back is a easy solution.
Sadly, the D-Pad is somewhat mush. While it isn't worse than the Xbox 360 D-Pad, I don't think you would have a satisfying experience with it if you were planning to play a platformer or virtual console game.
The ABXY button layout is the same as the original gamecube controller. A and B are glossy see through plastic while X and Y are your basic plastic. A and B give off satisfying clicks while X and Y presses are similar to those of a gamecube controller. Also, A and B are similar to those of PDPs afterglow controller buttons.
When playing Project M, I definitely had issues as the controller just felt weird at first. With the different shaped triggers, the joysticks at different elevations, and the glossy A-B buttons It tooks me a minute to adapt. However, when I did I found this more useful than any Classic Controller Pro could be. Aerials though were somewhat tricky to pull off with the C-Stick being replaced by an actual analog stick. It was something I was used to with the Classic Controller Pro, so the size difference kinda threw me off. Those who have used and like a GC Controller will be thrown off completely from the analog sticks probably due to everything being different.
Mario Kart 8 was really simple to pick up after playing Project M for a while. I would even say it's the most comfortable way to play it at the moment.
So overall, it's pretty good. Bare in mind that I don't like the original gamecube controller for Smash. I find that my fingers can't grip on the Joystick and full press L and R buttons hurt my index fingers after time. However, I do see the other side of the spectrum where people have been acustomed to the GC controller for Smash. With that said, I don't think those people should pick up the Wii Fight Pad. The changes are possibly too radical and it will come off as weird to them similar to how I thought it was at first. However, for everyone else this is a good controller to pick up. It has more going for it, IMO, than the gamecube controller.