The Gamecube was blessed with a great controller, a topic that I have gone into detail about before. But, the trouble is, it's only for the Gamecube. It's restricted to one console, and you can't use it on other consoles, or more importantly, your computer. Thankfully, there's a solution for that. Various adapters have been made to do this, so whether you are emulating the Gamecube on your PC, or just prefer to use it for games, a USB adapter is a good solution [your only solution, really, as opposed to building a custom controller, or a dedicated Gamecube USB controller].
As I said before, a USB adapter for a Gamecube controller is particularly useful for Gamecube emulation on the PC, and hopefully soon, on the PS3. If you have a powerful enough computer to handle it, and don't mind emulation, then you have what is effectively a portable Gamecube. You can even use retail Gamecube discs in your DVD drive, if you're touchy about piracy. The secondary purpose of the USB adapter is to use the Gamecube controller for other systems. While this is normally achieved via emulation, but the PS3 can also use USB controllers. Unfortunately, it doesn't work very often or very well for most Gamecube controllers, but it is fun when it does work. Using the GC controller for emulators in other systems is a delight, for the Nintendo 64 in particular, where you can substitute the C-Buttons for the C-stick, and on the PS1, where you can use the GC's analog sticks in place of the Dualshock.
So, if you're interested in using a Gamceube controller on your computer or your PS3, then give the adapter a shot. If not, then no need to go out of your way. I would only get it if you can think of a good use for it.
This is Lisalover1, hoping this didn't sound like an advertisement.