It works exactly the same as a SD Gecko, the only difference is that this one can easily kill your Wii and disables you to use the Wii SD slot a a Wii one. You said you was able to savve your GC games to your SD? Explain...
Any mods that you do, you take a risk and I was willing to take that risk.

If you have good hands, follow instructions and also double check your soldering to make sure its making good contact and its not bridging to any other joints.
I haven't tried GC games as I do not have any to try and I bought a Wii to only play Wii games. I'll try one later but for the mean time all i was worried about, was to create backups of original Wii games so I could store them and not get them damage by my kids!

Like the Tutorial said on the official site, you take the SD card in/out when you want to use it for either GC or Wii SD - simply loving it.