I got the Sony A7III some weeks back but due to pre-holiday workload, I haven't been able to really get used to it.
I have earlier experience from an A6000 which wasn't all that positive despite the promises it held.
Now my first impressions: The product is mature. The feel of the camera is solid. You can get it set up exactly as you like which has its pluses and its minuses. The choices are overwhelming. I followed a tutorial from Wim Arys Photography which he has taken down since.
Coming from Canon 5D4, it takes a while to realize that I am looking at a screen, not the image through the lens.
The Tamron 28-75 is sharp and luminous but of course not the same overall quality (corner sharpness) as the 24-70 f/2.8.
The overall size of the combo is slightly smaller. Laws of physics set their limits. The buttons on Sony seem to be made with nimble Asian fingers in mind. There is a clear improvement in accessing the menus compared to the A6000. For the time being, I prefer Canon's user interface. This may change with time.
I could not get Samsung A8S to connect through wi-fi. With iPad and MacBook Air, I had no problems.
My first impression about usability is that I am on the steep part of the learning curve.
On the computer screen, the RAW pictures from the 5D4 and the A7III are indistinguishable except when it comes to getting details from shadows. This should not be news to anyone.
The Sigma adapter is still on its way so I do not know how my Canon lenses will behave.
In addition to the Tamron, I got a Zony 55mm f/1.8, the Rokinon 35mm f/2.8, and the 100mm f/2.8 macro.
Upgrade: The Sigma adapter arrived. The autofocus hits it right away (EF 16-35 F/4). This is totally another game compared to A6000 with Metabones that was like Bono of the U2: "Still cannot find what I'm looking for". I sold my 5D3 to a colleague of mine. The 5D4 is still there.
Now I have a free weekend. Will be posting pictures.