This controller adapter works flawlessly when used with Super Smash Bros Wii U. I've tested it with old Gamecube controllers, the newly releases Smash Bros. Gamecube Controllers and even Wavebirds. They all work perfectly.
At only a fraction the price of Nintendo's first-party peripheral, this is a no-brainer. What's more is that you can flip a switch on the back of the device and use your Gamecube controllers on your computer. I've tested with Win 7 64 and it works without any issue, and without needing to install drivers. Boot up your emulators and go.