Agree with the two previous comments.
My D3S was a huge upgrade from my D3.
I bought a Sony mirrorless to lighten the load but I just can"t seem to part with my D3S considering the low price they are going for right now.
It's worth more to me as a camera I can use when I do not need to go that far from home.
If you haven't shot with one of the pro bodies it is a real joy. That said if you are used to the AF in any of the current gen of bodies it will be a step back in that regard. I shoot a D4 and the AF in D500/850 are far superior.
I agree with what's been posted. For pure image quality, there's several Nikon cameras that have been released since that beat it. But in the complete picture when you look at image quality, high ISO capability, frame rate, buffer, focusing ability, build quality, etc., it's really only been beaten by the D4s and D5 (and maybe D850).
To go off wfhat @Jonathan F/2 said, I've loved the D750's I've used, and if you took a picture of the same static subject, the D750 one is probably going to be viewed as better by most. To me, the pictures that camera produces straight-out-of-camera are sharper and have better color and more accurate auto white balance vs. the D750.
But when I looked at buying a D3s or a D750 3.5 years ago, I opted for a D3s and haven't looked back.
The photographer at the newspaper I work at bought a D750 in the summer of 2018 (he physically can't stand to handle pro bodies due to back issues, so he's never owned one). It's worked well for him -- but he's been frequently frustrated by the D750's small buffer many times and wishes he had a lot more FPS for sports and action. Plus, it hasn't held up well for him -- rubber grips are barely hanging on and are warped. I think that issue is mostly due to how he handles/abuses his gear, though.
I'm parting with my D3s, though, because I bought a used D5 last month -- two days before all sports ground to a halt. Got to use it twice at prep basketball tournament games, and it's been sitting in the case since.
So, uh, cough-cough, I've got a D3s if you're looking for one...
Big D3S fan here. In my opinion it's better than my D800 in every way possible, and also better than the D3 I was using before it. I was really scratching my head when I got the D800 because everyone was losing their minds over it when it was new as if it was some revelation. This "latest and supposedly greatest syndrome" leaves me very skeptical of the D750. No personal experience with a D750 so take this with a grain of salt but I've seen reviews testing the D750 AF system and it doesn't appear to be any better than the D4 auto focus. Not to say that's not incredible because it is incredible. But not better. I know for sure that the D4 AF is better than the D3 AF as I've compared them directly but from memory, the D3S AF seems better than the D3 (no direct comparison). I just don't miss shots with my D3S like I would with my D3. What bothers me about the D800 is that the D3s focuses better in good and even bad lighting and it's not supposed to. Don't get me wrong, the D800 AF is great, but the D3S AF is almost always flawless.
Ok, rant over.......
Point being, if you're not a pixel peeper, and only check to make sure you nailed focus, the D3S is a beast. Fast, great autofocus, beautiful files, tank like durability, insane battery life. I can get D3S files to look just as amazing in Capture One as D800 files with better ability to save high iso files. The D3S files don't lose their "Pop" at high ISO like the D800 files do.
I'm using my D3S daily and my D700 from time to time. I'm never disappointed about them. And for the average pictures 12MP is enough.
I borrow a D850 rarely from a friend if I need higher resolution. But I can't see a date to buy one soon.
The D3S works and works and works...
And it is build like iron.
Perhaps I'll change it for a D5 - but not necessaryly.