I did the Prophet 600 GliGli mod last year and it did wonders for my Prophet. The instructions on the website are clear and straightforward for the most part. The programming was simple copy and past and the soldering was very basic. The tricky part for me was getting the original chip out of the Prophet as I didn't have the right tool - an IC Puller. I had to work pretty hard to get the chip out without damaging it. 🤦 But once it was out, popping the Adafruit Teensy++ in was easy.
The documentation and manual for the new features are sparse. I wish the programmer had gone into a little more effort to explain the functioning in a user friendly way, but this is not a beginner's project anyways, so I guess you can't have it both ways. They did go to the effort of recreating all the original presets, which I can agree sound very similar. There is a bit of me that worries that I will be in a jam if something breaks, but I do still have to original chip somewhere.