They created the d-pad, invented shoulder buttons, miniaturized analogue sticks... They're the people who keep Sony and Microsoft up at night, by constantly re-inventing their interfaces. The Wii continues this tradition of innovation with perhaps the most radical departure from standard controllers since... Well, since forever, but the GameCube represents Nintendo's most advanced traditional controller.
First impressions are very favourable: the controller sits nicely in the hand of adults and children alike thanks to the narrow handles - only two this time, a nod to the superior (and cheaper) 2-handle design pioneered by Atari and made popular by Sony - instead of the trident configuration used on the N64.
Designed for 3D games, the primary left-thumb input is an analogue stick, with the secondary a d-pad. This latter device is perhaps the single point of failure for the GameCube pad, but we'll come to that in a moment. The analogue stick features something unique to Nintendo pads: an 8-point gate, making it easy to hit the compass points accurately. It's really strange no other company does this, as it is superior to the round gates of other pads.
The buttons are very well implemented, and this is where Nintendo trumps the competition: they gave the most thought to what works, tradition be damned. While it has only four face buttons (the new industry standard) each is unique in its shape, colour, size or orientation. New users require very little time to work out what button on the pad corresponds to what's shown on screen, 'cause every button's a little different. There's a giant, inviting green button, a smaller red button, and two cashew-nut grey buttons, one rotated 90 degrees from the other.
The secondary analogue stick evolved from the N64's C-button, inheriting the yellow colour and - somewhat strangely - the name: it's still called the C-Button. Unlike dual-analogue pads from Sony and Microsoft the GameCube's secondary stick is very obviously secondary: it lacks the mushroom-head top of the primary stick, making it sort of stubby.
Finally the shoulder buttons: without doubt the best shoulder buttons ever made. They're very large, and have deep grooves so your fingers find and settle into position without looking at them. With only one shoulder button on each side there's no need to look at which one is under the fingers: you're either in position or not. They have lots of travel for extra accuracy, and - uniquely - they also have a final 'click' as they reach the maximum position. This tactile feedback lets the user know for sure they've pulled the trigger all the way, as well as signalling to the GameCube that the maximum position has been reached. Oddly enough there's a bastard stepchild shoulder button as well: a tiny blue button, asymmetrically placed above the right shoulder, called simply Z. It's not often used, serving as a kind of last-resort button for games that need it.
If there's anything wrong with this pad, it's the small d-pad: Many players find it uncomfortably small, even though it's no smaller than the GBA or DS. For a console controller, it's definitely the smallest d-pad ever. Luckily not many games require this as a primary input, and for those games that would most benefit from a real 2D pad, Hori released a SNES-alike pad for the GameCube which is absolutely sublime.
The GameCube pad is not a panacea, but no pad is. Four face buttons mean six-button fighters can't easily be played, and even for four-button fighters (Tekken, Soul Calibur) they're not symmetrical, so players must adjust their reflexes, and the d-pad is too small for primary use. In all other respects though, from design, accuracy and build-quality, the GameCube is top notch, and probably the best analogue controller ever made.