I am working with the EOS R5 now since the 30th July. The camera is just amazing! For stills I am chocked with the performance of the sensor, it delivers such pleasant and detailed photos! For me it´s the best camera camera in the market!
For video, I must confess that the first use was a chock! I got overheating pretty soon in a hot day when I was shooting stills first and then video. The main problem is not the record times it´s that the camera is always loosing record time when in use, even if you are not recording. But I record in Atomos externally so the only big deal was in 4k120fps. After the firmware upgrade i am getting much better results and after that I didn´t had overheating anymore. Also the days are now colder in my country so tht may help either.... Last week I was filming vultures in a project and I was using 4k60fps recorded in atomos and 4k120fps internally in card. I filled the card and continued recording in atomos. I also made some photos. No overheating. So I guess camera did improve and if you use the atomos you will probably don´t have issues at all.
So far....I am in love with this camera!!!
If you want to check the vídeo, recorded with 4k100fps and 4k50fps in C-log, neutral, with just a bit grading. I used Canon Lut for the 4k100fps but for some reason the lut doesn´t work good with prores files that came from the atomos so I had to match them by hand. The image quality in video of this camera is outstanding!!!