Well after some disappointments with the Skillz Gamecube to USB adaptor(force feedback crashing my computer, weird controller response, didn't work with some games), I decided I would try the Super Joy Box 13. This thing works a lot better for sure. The configuration screen is simple and also I don't have any random button presses like I would have with my other adaptor. It works great with any game that supports Direct Input(which is pretty much every game nowadays). I have tried Prince of Persia, Halo, Need for Speed Underground, Silent Hill 3, Final Fantasy VII and even Battlefield 1942(for the vehicle controls). Force Feedback works very well and the controller doesn't have any problems with my games. MayFlash did a good job with this one.

Woah, nice piece of hardware. The driver software is very well designed and easy to install. And that's something in which companies who make these devices fail very often; crappy and buggy drivers. But as I said, these drivers gave me a very good impression (the GUI design says a lot about the inner workings of the drivers in my opinion). Also the support on their website (http://www.mayflash.com/) is great. Driver downloads, FAQs, product details, it's all there.
 
I only tested this device in emulators (it's all I play with, apart from my NGC), and it works perfect. Rumble features as well.
 
I even tried my PSX > NGC adapter and it works as a charm as well! Even the rumbling!
 
The boundary translation/octagonal movement shortcoming that Corrodias wrote about is there indeed, but I haven't found any real disadvantages in gameplay. The funny thing is that if I plug in my PSX controller (dualshock) the boundaries aren't there, and the cross in the calibration dialog fully moves to the corners. It seems to move faster from one end to the other as well, compared to the NGC one. So I'm not sure if it's a driver problem, just some oddity in the NGC controller itself. But let's hope it will be fixed/fixable.
 
Other than that minor flaw this thing is perfect and especially great for emulation fanatics So, go and buy it, and enjoy playing all of your PC games/emulators with the best controller there is; the NGC one.

I own a Super Dual Box myself (PS2->USB) and it works great, no hassles, absolutely love it.
 
A friend of mine really likes his Gamecube controllers, so I decided to get him the Super Joybox 13 (This converter).
 
Saddly it does not work nearly as well as the PS2 device.
 
The biggest problem? - Configuration. Yes you can calibrate it through the very nice looking GUI (control panel). However, what you can't do is enable/disable certain buttons. Why would you want to?
 
Well, one problem lies with the shoulder buttons (l & r), since they are analog on the cube, the driver sets them each up as an individual axis (one direction on a 'stick'). Thats fine and dandy, except that axis' essentially have 3 values, positive, negative and zero (pushing forward, pulling back, and neutral). This doesn't translate well into the gamecube shoulder buttons. By default (and there is no way to change this) when the buttons are not pressed, they are considered in the driver to be negative (pulling back). Which means if you aren't pressing the buttons, it's the equivalent of having a stick pushed all the way "down and to the left" *all the time*. As you can imagine this causes severe havock in many games. The only way to keep these values "neutral" is to press each shoulder button down half way (which is not fun!) for the entire time you want to use the controller.
 
If a game lets you configure the controller itself, you can sometimes get around this (by disabling l & r) but alot of games won't do this since l&r are considered axis' (ie. a virtual analog stick) and are often hard-coded by the game to commands.
 
This in my mind is a major oversight on behalf of the designers. The default value for a button/axis should never be anything other than neutral. L&R shoulder buttons don't translate nicely into an axis' style input. Instead they should either have been set up as rudder/pedal type controls , or just ignored the analog part, and made them standard off/on 'click' buttons. At the very least, they should allow you to "turn off" l&r in the driver control panel, to at least make the controller functional
 
The way it is now, the controller is mostly unusable