Now is one year that I owe the Sigma 500 f4 sport, and I decided to buy it because in that moment, starting my bird photography passion, I wanted a F4 500 and the price difference with the nikon f4 was enough, along with the reviews, to make me do the step.
Is a heavy lens as you know, but can be handled , and I forget that hard work to carry it, every time I come back home and I open the files.
When I bought it I had a d5, and the combination was honestly something remarkable ; sharpness, colors, bokeh don't make you think to want to try to buy the more expensive Nikon as an improvment, because was good enough what I had.
Has two setting of AF performance, one is very fast but a bit less precise, I noticed that gives less results on focus in burst shooting with the d850 while with the d5 was mostly fine.
I did not ever tried the Nikon 500 f4, so I cannot compare the IS, but works fine enough for me.
I decided few months ago to buy the 500 pf because I'm growing old and I thought was a good idea to enjoy a lighter glass.
Well, on the d850 I couldn't see the difference in sharpness, when there was , it was me don't hold the big one solid as I should. AF same story. Did not notice as well any difference in lock the bird in darker zones. Being so small, with the IS engaged, the PF can goes lower in shutter speed because is easier to hold still, but the difference is always 1 stop ISO, so pretty much same story.
In the meantime I decided to buy to add to my d850 and start to move to Z, the Z7ii. Both AF speed drop very badly in my opinion, is very disappointing, when you are used to have fast AF lenses, to see that. I'd take a little advantage (but is so little) to the Sigma in the faster AF setting. The sharpness of both on the mirrorless is crazy.
Because the only advantage was the weight, and tones passages for me are better on the Sigma, and because I'm really addicted to the separation that gives the F4, I changed idea and kept the Sigma, and decided to sell the PF, and give a go to the Sigma once the Z9 will be here. If the Z9 won't push the AF of the Sigma at least as same as was going on the dslr, it will be time to change it too.
I feel to can reccommand this lens if you are keen to carry it, and if you are using a dslr, get the Sigma dock station. I did not need to fine tune it on my d850 but some people had to.