Let me shared my review as I have been invited by Canon to test the pre-production R5.
Autofocus
Canon EOS R5 eye focus had improved tremendously.Once set to eye focus, it would lock to the eye even if the person walk away, or briefly blink or turn away from the camera.
If you don't mind to drain your batteries you may even leave the AF to servo mode all the time.
For normal usage the AF is accurate enough even though you forgot to switch from Animal to Human. Since it's using an adaptive learning, it's better now to always leave Animal AF if you don't intent
to shoot Animal as the machine will learn and store the wrong setting to Animal.
Focusing is a challenging lighting condition where the previous eos R would struggle, the R5 perform flawlessly.
It's able to track fast moving subject even with the default setting. To date I couldn't find any situation whereby the R5 could not lock focus or missed focus where 1DX mark III could.It's seemed like even without a dedicated AF chip as with 1DX mark III, it's already good enough. The AF performance placed it on par with the Canon flagship 1DX mark III. The high fps finally made it suitable for fast paced action photography. Alternatively you may shoot 8K and crop it out provided your event last before the overhead icon pop out.
ISO performance
In term of ISO performance, ISO up to 12800 is very usable, almost noise free as if you're pixer peeper. ISO 25600 is usable and clean enough to use.This in turn finally placed Canon at least on par with Sony high ISO performance. Note that I had turned off the Noise reduction.
Handling
With the addition of the thumb-wheel and multi-controller joystick, added a dedicated magnify button and Q button to access the menu.
quick dial. Finally we have a mirror-less camera that has the same handling as 5D. 5D user would easily pick up the handling of R5 compared to R.
IBIS
Depending on the lens, the IBIS is really a useful addition to R5. Low light shooting without a tripod was made possible with IBIS and high ISO performance of R5.
As for the video with IBIS, it's good enough for casual or blogger to use compared to R.
Some improvements.
The viewfinder sensor is a lot smarter. The rear lcd screen of the EOS R would dim if we accidentally cover the EVF sensor.
The R5 EVF sensor is a lot smarter. The higher resolution of the viewfinder and the higher refresh rate bring it almost like using an optical viewfinder. In my opinion the shutter sound nicer, perhaps due to higher fps.
Video
The camera will overhear if 8K video is set. Setting Overheat Control function to “ON” will prolong the shooting time but will still get overheated as due to the small size head dissipation is not that great but we're talking about a compact 8k camera!
The built in zebra pattern is very useful for amateur as we don't need to use an external field monitor for that purpose.
Summaries
Canon EOS R is the first Canon full frame camera. R5 improved where R lacked of. We are actually getting a 5D mark IV ,1DX mark III (I personally believed there's a reason why Canon used a dedicated AF chip for 1DX mark III but the AF performance for R5 is good enough for sports photography) and 5DS resolution. It's a body that could exhibit the RF lens performance. Even if if you use the cheaper RF lens, the quality of the RF 35 f1.8,RF 85 F2 stm is cleaner and the kit lens, 24-105 than the EF counterpart.