This review is coming from the perspective of a long time Nikon user who has also used Canon in the past. Like a lot of other people, I was intrigued by the potential of better auto focus with the Z9. This obviously applies more so to people that like to capture photos of moving objects (vehicles, animals, people, etc.). In my time with the camera thus far, I have photographed people running, jumping, walking, and dancing. A lot of my work also includes portrait photography, and some of that involves freezing a movement versus a person always standing stationary. While I am able to capture a lot of what I want with the Z6II, the Z9 allows me to capture a higher number of keepers under the scenarios mentioned above with similar camera settings. One of the other reasons I was interested in the Z9 was for video purposes. The 8K functionality is cool, but it is something that will be used more in the future and not right now due to the storage needed to work with hours of footage. My bigger interest was internal 10bit H.265 (SDR/N-Log), Prores, Prores Raw, and 12bit N-Raw. As I eluded to earlier, 4K is plenty good enough for right now. Each format has advantages and disadvantages I will get into a little more below. As a whole, it has been a great camera. The colors look accurate under most scenarios with proper white balance. Photos are processed primarily in Capture One, but I still use Lightroom sometimes. Keep in mind that as of this writing, Nikon's high efficiency raw files are not supported in Capture One yet, but I'm guessing it will eventually be supported. You'll need to use the larger format or do all of your editing in Lightroom instead. For video editing, you need Davinci Resolve Studio to edit N-RAW, Final Cut Pro to get the most out of ProRes RAW, but it can still be opened in Premiere if you are only an Adobe user. As a small note, N-RAW is way smaller than Prores RAW for 12bit internal video. The camera as a whole is much heavier than the Z6II, but if you are coming from a pro level DSLR with an integrated battery grip, it won't be a big deal for you. There is a lot of customization options, say take your time to learn what you need for either the photo or video work that you will be doing. I have been shooting a lot of photos and videos with San Disk and Lexar CF Express cards with no issues. I've used the camera with triggers from Godox and Wescott with no issues. My Atomos Ninja V connects with a full size HDMI, but it won't record ProRes RAW if you want to avoid the high cost of expensive 512GB high performance CF Express type B cards. It does work well as a monitor, viewing false color, and recording ProRes if you want to use it for that purpose.
Video: The waveform option for video has been a useful addition. I've been able to take pictures for 30 minutes, shoot 1.5 hours of continuous 4K 24p footage without it overheating, no error messages, and more before only bar disappeared from that battery. I burned through one Z6II battery doing similar tasks for a similar time frame. In general you can shoot over two hours of continuous footage in one take, assuming you have a big enough card. A 256GB card will get you at least two hours of H.265 10bit internal SDR/N-Log at 4K 24p. I believe N-RAW at a similar resolution, but 12bit, will be over an hour, but 60p will be less.
I'll provide more feedback on photos and video after another month or two of usage.