The Phase Fresnel lens allows for an extremely lightweight and compact design at least compared to other lenses with 800mm focal length. Nikon has already used this special type of lens for two other extremely popular lenses, namely the 300m f4 and the 500mm f5.6. Both lenses for the f-mount, the lenses are nevertheless exceptionally light, compact and yet razor sharp.
At just under 2.4kg, the 800mm is indeed a lightweight. Even with the relatively heavy Z9, I was able to shoot handheld for longer periods without a problem. What is especially noticeable compared to the 400mm, however, is that the lens is less well balanced for me personally, as much of the weight is far forward on the lens. This is especially noticeable because the manual focus ring is very far back. Since I personally still use manual focus quite a bit, this bothered me a bit. So I had to hold the lens quite weirdly in my hands to balance it well enough, but still get to the focus ring.
While we're on the subject of the focus ring, it has quite a bit of resistance and to get from infinity to very close, you turn for what feels like an eternity depending on your speed. The fact that the focus ring is so far back doesn't help with the whole matter either.
Further to the front of the lens is a function ring, as found on virtually all Z lenses. And like on all other Z lenses, this was pretty much the first thing I disabled in the menu. So far, I haven't really been able to get comfortable with this ring (and I've tried a few times).
I would prefer - as with the 400mm 2.8 - if the function ring and focus ring had swapped places. The Z9 offers the ability to swap zoom and focus on certain lenses. Maybe there will be that feature for the focus ring and the function ring someday.
The fact that the resistance and the position of the focus ring (something that is hardly needed with the Z9's focus system anyway) are the only real points of criticism shows how well the lens performs otherwise.