I stumbled across this looking for some way to get Crystal Clear on a cartridge. Picked up one of these and it works perfectly. It doesn't look as cool as the non-rtc flash cartridge, but it works perfectly.

Got this for the recent translation of Pokemon Green. Works perfectly, and the PCB looks really cool.

This is perfect, the only way it could be better is if it could be flashed directly (i.e. if it had a microusb and the flashing circuit integrated). But the PCB looks quite full so it does not look easily feasible. The look and feel is that of the original cartridges for GB. Awesome!

I'm totally satisfied with the product itself, it works out of the box with the provided drivers and software. No problems so far, I tried flashing 7 games and they all worked. It would be great to have a documentation saying that you need to select RUMBLE to write the flash (that's the only one that worked for me so far) and that you need to erase the flash before re-writing it (no overwrite). You also need to select the size depending on the FILE SIZE you want to flash and not depending on the memory available on the cartridge (I bought the 4MB one and tried to flash games using 4MB rather than 128kB). Without documentation, even geek users will spend some time figuring out how it works because the attempts take time and if you don't chose RUMBLE you need to disconnect the cart and reconnect.
Anyway a great product, the 3D model of the case can also be downloaded on thingiverse and printed on any 3D printer.

Exactly as described, the game cartridge works perfectly. Excellent customer service too: the seller was quick to contact me regarding what ROM I wanted, and even designed and printed a custom label! International shipping was surprisingly quick. A smooth pleasant transaction and a top notch product, thank you J. Rodrigo!