Received my adaptors, some thoughts:
* Only tried the SMS one so far. It works beautifully

* Took the SMS one apart, and there is indeed space for a GG slot. I'll have to try this... it's slightly annoying that the holes were pre-filled with solder, as it obviously makes it harder to add on, but hey.
* Took the Game Boy one apart, was surprised to find the N64 slot still there! Is it functional? Is there a mod I can perform to make the cartridge detect switch work? I do like the inset cartridge socket, though, makes it look like a small Game Boy player for a Mega Drive

* No surprises under the case of the N64 adaptor, though of course, it's still well-labelled and good-looking.
* The build quality is amazing. Only Matthias could have done it this well, with this much attention to detail down to the Retrode engraving on the back. I do love the economical plastic bags with manual in which they came, as well; they seem to be just the right size - I like minimalistic packaging.
I only think it's a shame the N64 one couldn't have had a bigger label - I have the same qualm about official Mega Drive J-Carts though; something about carts with controller ports on them just seems intrinsically unofficial to my brain. No way it could have been improved
