The R52 is and feels as tall as the 5D3 but significantly smaller. My small hand rests perfectly on 5D, taking the R5 in hand gives me a more "aggressive" feel which I actually don't dislike because when I lift it I know I am taking a shot. It is also easier to "play" with the body a little bit while keeping it close to the eye, which is something useful when taking pictures of people since it makes them feel less targeted, it is also generally less noticeable. The smaller body will be bothersome for those that cannot get the full height of their hand on, or those that want a "professional" look for clients, but there are many choices for vertical grips so it's a non issue. One thing I do not understand is why the current grip pattern doesn't cover the full extent of the plastic as it does on the 5D. I tried the R1 as well, I do like to have that in hand and the cross grip pattern is better, but it's just too big of an object to push in someone's face, let alone the price difference. The buttons of the R1 are also better, but R5 is still excellent, I just had some troubles to reach for the joystick but I think it is just a matter of habit. The new third wheel for ISO is great. The general balance while using the EF adapter is perfect.

I took a lot of shots so I can give my opinion about it. I had read a lot of different things about it which had scared me. I know the camera is way more complex than DSLR and there is a lot of code powering it. My 5D also had some hick-ups, they are not perfect either, regarding the R5 I think I benefited from being able to jump on the most recent firmware that corrected a bunch of issues and it has been generally very reliable for the amount of pictures I took, there were two sets of issues though. First thing is a random inability of the camera to take pictures, it happened randomly around 3-4 times in 45 days and lost me one picture, to be precise. The camera would respond properly but there was no flashing white border (I am shooting in blackout free mode) when pressing the shutter and the picture was not taken. It came back working after around 10 seconds. I did not power down and up the camera but I guess that would have fixed it faster. It would be nice if Canon could prevent something like this from happening ever but maybe it's not something realistic, I don't even know if the reason behind the multiple occurrences of the bug was the same. The second set of issues appeared during a rainy day and it was way more concerning in my opinion. I was walking and shooting under an umbrella, being careful to wipe excessive water that eventually builds up on the body and viewfinder, I wouldn't call it a very challenging scenario. Still the viewfinder completely froze two times. It freaked me out a bit because I was afraid something serious could have happened with the water but I doubt that's the case because since it came back working as normal, I was also being really careful, I don't see how water could have passed the tropicalization. Maybe an issue with coldness plus water? It wasn't that cold though.

The reason behind my upgrade wasn't picture quality but still this is such a relevant topic that I ended up caring about it a bit. Let's start by the jpeg presets. I really love to shoot raw+jpeg because I do not care or have the time to edit most of the shots, I want them to look good out of the camera and Canon cares about that and it kept improving on what it already did. I used the faithful picture profile with minimum contrast on the 5D and kept doing the same on the R5. I was very pleased to see how the shifting reds are now fixed, it looks perfect now and I can trust it to deliver a realistic image. I am also extremely happy about how the camera deals with the data from high ISO. I have let it go up to 6400 and there is a night and day difference with the 5D, before I was scared as soon as I would hit 800 ISO, now I am thrilled about the night time results of 6400 and I've heard people pushing it even to 12800. This really allows you to take shots that were not possible before, keeping the aperture close, not using flash, fast shutter speed.
I have not much to say about resolution, I liked the 5D already, the extra pixels don't change much for me but I am not complaining. I do hope that we will get to a stable point though, bigger numbers each year must come at the cost of speed and other features.
My main complaint about picture quality comes from Dynamic Range. It scares me to read comments that say that dynamic range is so good today it doesn't need improvements, the reality is that dynamic range today still sucks, it is unbearably bad. Digital photography on the top class cameras is still unable to cover a sunny day with some shadows, how is that good enough? We need at least 3 more stops of DR to get there and it doesn't look like current sensor technology can improve. A breakthrough can come with DGO but it has to be implemented in electronic shutter mode for me to be relevant. I would gladly sacrifice burst speed to get it but I doubt that Canon will implement such a significant quality split and will instead wait to have it working perfectly for all modes. Just to be clear let's look at this example:
Normal sunny day, one person is shielding their face from the sun and is correctly exposed and one is not and is completely blown out, no details whatsoever to recover from the raw file. Terrible stuff which would never happen on film. The shot was taken at 800 ISO, a double gain output 100/800 would cover exactly the 3 stops needed to make the image decent. This is such an important aspect for me, I feel like a lot of people just don't take enough wide shots with patches of light and multiple subjects and are instead isolating a main character or shooting in interiors so they don't see the issue. It's baffling that we got 30 photos taken in one second before being able to shoot a sunny day.

This was my main concern coming from DSLR. I had tried some digital viewfinder on lower end models and it looked unusable to me. The R52 looks very very good, it's better than shooting with an OVF as well, especially at night where there is no match. With satisfaction my expectations rose as well and I noticed that I am not able to fully perceive faces expressions while looking through the viewfinder. I tried with an R1 as well and while I do notice the extra resolution and bigger magnification there is still something missing. I don't know why. Are my eyes not focusing well enough? The text looks crisp though and the 120 frame rate seems smooth enough but I just can't parse expressions as well as I do with my own eyes. Any improvement on the viewfinder front will be appreciated and is to be expected, but I am happy as is.
Now that the quality of the images is very similar across the lineup the viewfinder is one of the main segmentation feature that companies relies on so it kind of forces you towards the 5 series.