I got tired of having to carry around a gigantic and also really heavy 150 - 600 mm lens (2 kg) and, therefore, decided to get this Canon RF 100-400 lens to complement my 150 - 600 Tamron. And I must say, I am very impressed!
The image quality is a lot better (as is the focus) and cropped images look about as sharp at 400 mm (if not sharper) than 600 mm uncropped images before.
Definitely much more enjoyable just walking around / going for a hike with this kit. I barely notice that I have my camera with me at all!
Here's a quick test image I took yesterday. Nothing special, just for testing.
Gave up on the Sigma 150-600 I have. Dropped to either a 300 with a teleconverter or a 500. The prime lenses are so much sharper. I'm in the Nikon side of things....