Hi Colin,
Still very much early days and what with bad weather and now lockdown, the opportunities have been fairly limited to get out and about so far but my initial opinion is that it's a fabulous piece of kit. I have never had so many gains in one camera upgrade since buying my first DSLR way back when! Most of the time in the past I have settled for one improved feature , maybe improved ISO performance, or frames per second but with this camera you get the lot in one package ( well maybe no improvement on ISO vs the 1DX2 but no apparent loss either) . I have seen lots of people say the camera is a game changer and in my opinion they are not over egging it either. Depends what you have previously owned of course and I have never had my hands on a 1DX3 but the R5 does everything my 1DX2 and 5D4 could plus a fair bit more too. Mirrorless is a new learning curve and it's injected a new lease of life in to my photographic interest. I love the fact that what you see through the viewfinder is more than likely what you get on the memory card, I love the layout of the camera, and I'm even getting used to the fact its miniaturised especially compared to my 1DX2. Probably the biggest gain is the fact the auto focus is amazing for tracking, it's not perfect but it's certainly a huge gain over my previous cameras. Not only does it track well, the focus points cover the whole of the screen.
The fact you can use your EF lenses with no apparent loss of functionality is a huge benefit too. I'm undecided on which if any of the current crop of RF lenses I might be tempted by but there is no hurry. The R5 can auto focus at beyond an f8 lens combination which is something my other bodies were not capable of doing other than in live view so a 2.0x TC on my 100-400 at f11 is doable. How it would perform with the RF800 and the 1.4TC would be interesting! The RF 100-500 looks to be a nice piece of kit and from past experience of using my 100-400, for birding it would always be at the 500mm end but I do think it's disappointing that if you mount a 1.4TC on it, it immediately limits you to 420mm-700mm which for bigger wildlife on say a safari holiday would be less use than keeping with my 100-400 which with the 1.4TC becomes a more usable 140-560mm zoom and doesn't cost me a penny as I already have that bit of kit.
The big question of course is will the R5 take better images than my previous cameras?
...... and the answer is No!
But getting them will be much easier now, especially the action shots.