I'm back from a wildlife trip to Jasper National Park, where I put about 9,000 shots on the R5 (along with the 100-500 RF lens). I also had my 1DXIII attached to the 500 f/4 with 1.4 extender, for the times when I needed more reach. Here are some observations:

R5 pros:

The 12 fps frame rate and buffer length were quite adequate (even for me), thanks largely to operating in C-RAW mode. I carried a couple spare batteries but the charge lasted more than long enough to keep the frame rate in the fast mode.

Ridiculously sharp. When the focus is on target, the images are crazy good. The high MP sensor is incredible, and you can crop to your heart's delight when you need to.

Good to 10,000 ISO, and beyond. Really! Sure, if you zoom in to 100%, it looks a bit harsh, but that's because you have so many pixels. When you're looking at it 'normal' size, it is fantastic. The tree hugger shot that I entered in the IOTW contest is ISO 10,000 and I only cleaned it up with a touch of luminance smoothing in Lightroom.

Quick switching from full to crop mode. I believe it was Joel who gave me the hot tip to program the DOF preview button to switch to crop mode --- what a super benefit that is. If the target is small in the frame, crop mode makes it easier to put the focus dot on it.

Super light and easy to hand hold (with the RF 100-500 zoom). Both the other guys I went with had 400 mm f/2.8 lenses, which gave them great shots and low light performance, but they just couldn't swing their rigs around as quickly or hand-hold them as long as me. So I didn't miss much when the action was underway.

R5 cons:

Okay I officially don't like the single-point focus. The focus point is way too big. We were photographing bear cubs, and even when I put the focus box right on a cub it would often miss and grab something else like adjacent grass. I tried switching to spot AF to make the box smaller, but the performance diminished noticeably in the low-light conditions we were often in (the lens would hunt back and forth). By comparison, the regular-sized focus box on the 1DXIII is much smaller, and when I put it on an animal, it grabs the focus right away and never ever loses it. The focusing area is smaller of course, but as long as the box is on the animal, it will always (always!) be in focus. Not so with the R5.

The 2-second startup time when the camera is in standby mode. This drove me utterly nuts, even though I trained myself to half-press the shutter while I was bringing the camera up to my eye to speed up the process. When the action starts up, you need to be able to respond quickly. And holding a blacked-out viewfinder up to your eye when cubs are running around in front of you is incredibly frustrating. I sure hope the mythical R1 has an instant start up. Or else I'll just disable the auto-idle timer and let the battery drain down.

The EVF. When I would grab the 1DXIII to take shots, I just adored being able to see the animals in 'real life', rather than on an EVF screen. It really is a huge difference. For this reason alone I know that I will always have a 1DXIII in my bag.

I'd be interested in hearing from those who have the R3, if these issues are also present in that camera. But overall, I'm super pleased with how the whole system worked.