The Fujifilm GFX 50S is an incredible camera. It's Fuji's entry into the medium format market and it is one hell of an entry. Using the same mirrorless technology as in the X series, the GFX feels like a DSLR only packed with one big medium format sensor.
If you're already a Fuji shooter, it takes only a minute to get it going. Same UI, very similar handling system. Only the mechanical shutter feels different when you take a picture.
I have tested it for both street photography and portrait, and the results were mind blowing. You can easily feel that you have twice the resolution. And the responsiveness is pretty good for such a big camera. For street photography however, you "lose" the "furtiveness" of a X camera body or that of a X100 fixed lens series, especially if you try to capture fast scenes on the move. As for portraiture, this is when you begin to dribble and check your budget. By the way, less than 10'000$ for the body and the lens, that's pretty low in the medium format market.
HOWEVER... Do you need that extra smoothness, these extra pixels, this extra feeling of the medium format? That's the question that should help you make the move. If your pictures end up in very high quality prints for editorial projects or simply art prints, then yes, that little luxury can be valuable. In my case, for now at least, the GFX 50S is far from necessary, and I can always rent it if really needed.
Here are two shots I've done for a portrait. If you pixel peep, you'll see the difference in resolution. Because the light set-up was the same, the aperture are the same (f/2.8 for the GFX, f/2.5 for the X-T2), making the GFX shot DOF shallower, a DOF that could have been pretty similar at f/1.4 or f/1.2 with the XF56mm. So to me, the result with the X-T2 and XF56mm (non-APD) is more than satisfying.
Now if you're desperate to get me one of those GFX, I won't say no, because the results are really amazing. ;)
But for now, I'm more than happy with my tiny APS-C gear... :)