I recently got a Canon RP, and am using it with adapted EF lenses. I wanted to try extension tubes with the RF-EF adapter, but did not want to invest in an expensive extension tube set since I was unsure of whether the RF-EF adapter would work at all with any extension tubes (I had prior experience with the Sony E-mount to A-mount lens adapters not working with A mount extension tubes, so I didn't want to take too big a chance on the Canon versions).
Surprisingly enough, this inexpensive extension tube works perfectly with both EF lenses that I have tried it on when used with the RF adapter on the RP. I was actually shocked that it worked flawlessly.
One other nice thing; the interior of the adapter has some flocking material on it to keep internal reflections to a minimum, which I think works better than the fine lines cut into the barrel interior in many of the other adapters.
Since this is an all plastic device, I would probably not recommend it if you are hard on your gear, or use it extensively, but I would heartily recommend it for light use. I actually will probably get the metal mount version at some point, since I do use it extensively, and know that it will work well now on my system, after trying this one. However, for the time being, I am happy with this and extremely pleased that it works so well with the adapted lenses that I am working with now.