It looks nice, but what does it do? I have a couple of old gameboy colours and a couple of the exact same cable you used but I haven't touched them since I got my NDSL.
Oh, and the midi interface is for syncing it up with other music gear. You can create 4 channels of sounds on the gameboy alone but if you want drum machines and other stuff...
Also besides sync you have the possibility to send midi notes and other parameters, and it can make the gameboy send midi-out as well. Haven't tried this out myself though...
Btw, an update on arduinoboy, Trash80 just released a new version with an accompaning gameboy program (mGB) that takes the gameboy midi (IN) control to a new level! Take a look at it!