I purchased the Nikon Zf with its 24-70mm f4 kit lens. Subsequently, I bought a few more lenses: Nikkor 14-30mm f4 and Tamron 150-500mm f5-6.7 zoom lens. Below are side-by-side comparisons of the Z-mount lenses on the Zf against the D750 and its equivalent F-mount lenses.

Zf and 24-70mm f4 vs D750 and 24-70mm f2.8.

The 24-70mm f4 is much more compact and lighter than its F-mount brethren, and also produces better image quality, and even though it is 1-stop slower, this is easily compensated by Zf's superior high ISO low noise performance as well as built-in sensor stabilisation.

Nikon ZF vs D750 LensesLorong Utara, Petaling Jaya, Selangor.

Zf and 14-30mm f4 vs D750 and Samyang 14mm f2.8.

The Samyang prime is a little more compact since it is manual focusing but it has a weird moustache-like distortion. It also has a lens hood that is not removable. The Nikkor ultra-wide zoom lens is certainly more flexible, and f4 is more than sufficient for general use, not to mention it produces far better images especially in conjunction with the Zf's sensor stabilisation that is optimised for the Z-mount.
Nikon ZF vs D750 LensesLorong Utara, Petaling Jaya, Selangor.

Zf and 150-500mm f5-6.7 vs D750 and Sigma 150-500mm f5-6.3.

Being a Z-mount superzoom lens, the Tamron's image quality, autofocus and compactness is much better than the Sigma. Although the Tamron is a little lighter than the Sigma, in practice, it feels quite chunky and dense.

Nikon ZF vs D750 LensesLorong Utara, Petaling Jaya, Selangor.

You can see a few more pictures here.