It may not be as good as the Nintendo controller, but InterAct's SuperPad delivers a very functional pad at a great price.
I really feel bad for 3rd party controller developers like Interact. It doesn't seem like they get any respect. Look at what they have to compete with. The GameCube controller melted into my hands in the first 2 seconds I held it. The control sticks, D-pad, and buttons were all within comfortable reach, except the Z button. The handles fit into my palms comfortably, even with larger hands (size 12 ring). Nintendo's GameCube controller is all-around their best ever, and arguably the best of all current consoles.
So it's little wonder why 3rd party controller don't get as much respect. They can't duplicate Nintendo's near-perfect creation spec for spec. They need to make variations to avoid patent/legal problems. It's not that all of the controllers are inherently bad quality, they just aren't normally equal to or better than the one introduced with the system, which have a larger amount of R&D behind them, not to mention big patents.
I tried out Interact's SuperPad on 2 games: Luigi's Mansion and Wave Race. My general feeling about the controller on the whole is very simple; the SuperPad is one of the best 3rd party GameCube controllers you will find so far. Is it equal to or better than Nintendo's controller? No. That wouldn't be realistic because of the design and legal issues I just mentioned. It gets the job done, and the design isn't so adversely different from Nintendo's controller that it ruins your game experience after using and being used to theirs.
The first thing you notice when you hold the SuperPad is that the handles are a little skinnier and longer than Nintendo's. The analog 'L' and 'R' buttons feel as if they're on the back of the controller, instead of on the top shoulders. The Control and C-sticks are larger and dip inward. The Rumble function can be turned on or off with a front switch, The 'B' Button is bean-shaped instead of circular. And the D-pad is a circular pad that has elevated bumps in the Up/Down/Left/Right positions, instead of being the traditional plus (+) shape.
Yes, the SuperPad is different than what you've been using. Does that make it demon spawn? Not by a long shot. It did take a little bit more time to get used to, but since I started playing on the GameCube, even the N64 controller feels foreign in my hands now. With a little practice, the SuperPad is a sufficient controller that can do the job almost as well as Nintendo's. In fact, I prefer the SuperPad's 'Z' button, as it has a more distinct feel and sound when pressed. The circular D-pad is a bit unusual, as the bumps aren't quite distinct ENOUGH that I'm certain I'm pressing in a diagonal direction when I want to. I can give or take the other features that make it different from Nintendo's controller. They aren't particularly better or worse, performance-wise.
For the cheaper price, Interact's SuperPad is a nice alternative if you're on a budget.