With the Lumafix64 it's possible to get rid of the vertical stripes in the video output of some C64s and improve the picture quality. It sits between the VIC-II socket and the chip, and two potentiometers allow you to fine-tune the picture quality. The removal of the vertical stripes comes at a cost though: while the picture will be much cleaner, it may also get slightly blurrier. All in all I consider the Lumafix is a great improvement, but it's a matter of taste.
The "Jr" version of the Lumafix64 is a very low profile board, which is excellent for the C64C with its slim case. I can still put a heatsink on the VIC-II, which would not fit into the case with a regular Lumafix installed. So I'm very happy with it, as it does exactly what I wanted!
The board arrived without any documentation - for me that's not a problem, because I already knew how to install and use it. I'm assuming that most people who buy this board know what it is and how to install it, but if you are a beginner, and maybe you don't know where the video chip is located and how to remove it from its socket, this might not be for you.
Inserting the LumaFix board into the socket can be tricky too, because some C64 boards have very cheap sockets. On mine it kept popping out multiple times, and it took a couple of tries and some force until it finally stayed in the socket. If you have the soldering skills, you may want to replace the old VIC-II socket with a better one ("round pin" sockets should be much better for this) before installing the LumaFix.