I really don't care for it on the whole, though it's not really a bad controller per se. When I first tried a GC controller, my prior controller experiences were the NES pad, the SNES pad, the N64 pad, and the PS1/PS2 pad. Most of my recent experience had been with the PS1/PS2 pad, so the first thing that struck me about the GC controller was how similar it was to a PS1 pad. But as I tried out Super Mario Sunshine with it, I started to find issues coming up...
First off, the L/R triggers: I was not expecting them to have so much give. Not devestating, of course, but it put me off guard as I was used to buttons being only pressed or not pressed. Next up, the D-pad: tiny and moved. I got used to an analog in the D-pad's old spot eventually, but I never did like how small they made the D-pad. Then there was the main buttons... Perhaps having a huge trigger with 3 jellybean-sized buttons around it works well for first-party games, but developers really didn't seem to realize that it's a lot harder to hit X, Y, and B than it is to hit A. Especially bad were the titles that required extensive use of B and X or Y at the same time. The Z trigger is really hard to hit, and should've either been made bigger or positioned better. And one on both sides would've been aesthetically pleasing, too. And of course, the C thumbstick is pretty awkward with that nub sticking up; I'd prefer a bigger analog like the main one has.
Other than all that, I don't have any really big complaints about the GC controller. It works fine, and it's not THAT bad, but those design decisions ultimately irked me. Definitely not a favorite controller of mine; too much competition from the SNES, PS1/PS2, and Wii controllers (and the Classic Controller too).