There are enough technical reviews of the Sony A7sii out there that I won't discuss the specs of this instrument. This is more of a reflection from the viewpoint of a working professional-- and one that is primarily a Nikon shooter, with a Pentax 645D as a secondary system. The first question you might ask is why on earth would I spend close to $3K on a Sony if I already have professional gear in two other systems?? It's not simply a matter of gear-mania. My work is expanding into video, so I wanted a tool that was specifically designed for that purpose. And the A7sii really is. I'll talk more about that as well.
Professional Craftsmanship
This does, actually, bear repeating. The A7sii is a expertly designed and manufactured camera. Its small size gives the sense of being unprofessional, but its quality matches its price tag. If feels rugged. While the small form has its advantages, I can't say I enjoy holding it as much as either my Nikon or Pentax. It's just small, ergonomically. I'll have to get used to it.
Each manufacturer manages to get something right that you wish the others would adopt: The Nikon is a fantastic all-around camera; the green exposure button on the Pentax is brilliant for working quickly in Manual Mode, and it seems like only stubbornness keeps the other manufacturers from adopting it. With the Sony, it's the abundance of well laid-out programmable buttons-- no less than 4, plus a FN (function) button for quick access to 10 more menu items. You never have to take your eye from the viewfinder. Everything is at your fingertips and shows up on the screen. I can't express how awesome that is.
Peaking and Focus Zoom. These are necessary tools for video, but are also incredibly useful for working with manual lenses in still photography. The viewfinder is sharp enough that you can get pretty good focus just by eye, old school, but focus peaking is quicker and zoom more precise. I purchased a K&F Concept Lens Adapter to mount my Nikon glass on the Sony. For 18 bucks I can use all of my lenses, albeit in manual focus mode only, and manual aperture as well. The adapter works well, though the aperture clicks are so close together they're impossible to count if you're trying to triangulate your exposure with your shutter and ISO.
Zebras. Again, this is great for video and still images alike. You can use the zebras to identify blown out portions of the image; or you can adjust them to identify when the skin is properly exposed.
Viewfinder. The viewfinder is nicely sharp and easy to use. The camera switches from the back LCD to the viewfinder when you put the camera to your eye, which is simultaneously brilliant and annoying. Annoying, because sometimes you'd like to turn off the auto LCD (to conserve battery or prevent your camera from lighting up). When you do, however, it won't show you the image when you press the Play button, which frankly is stupid.
Battery Consumption. This little guy chews through batteries like a pit bull. Sony includes an extra battery in the box, so they're obviously aware of the shortcoming, but even two batteries only gives me about half the life of my Nikon. If I find some power-saving tips I'll post.
Image Quality
Much has been said about the low light abilities of the Sony A7sii. So I needn't say more. Except I can't help myself. It really is two steps above my Nikon D600, and it's hard to believe until you look at the files just how well it handles near blackness. Between ISO 100 and 2000 there really isn't any advantage between my three cameras. But from 2500 to 40,000 the usability of the image is stunning.
This is at ISO 40,000. It's had noise reduction applied in Lightroom, but that it's usable at all is a miracle. Yes, I'm only lit by my cell phone; otherwise the room is black.
For professional use, 12MP is a little disappointing if you've become accustomed to having more to work with. It's not simply that you can't blow up the image for printing; you can't crop down very far either, and have it keep its quality. The image size is more than enough for casual shooting, but less than what I need for professional. For this reason, it won't ever become my primary photography camera. I do pack it as my backup.
The other aspects of image quality-- color rendition, dynamic range, etc.-- are quite good. I may have more to say once I've spent more time with it.
Video
I'll save a real review of the video capabilities for later, after I've spent more time with the camera. Others have said, and I'll repeat, that the A7sii is really designed for video, with still images as a back up. With most DSLRs, you have to rely on accessories to get all of the tools you need to capture footage efficiently... things like focus loupes, peaking, and zebras. Those are built in with the Sony. Plus Slog2 and Slog3 for professionally flat images. And the mics are much better. You never want to use your camera mic for talent, but it produces good quality for ambient sound and even for a behind-the-camera interviewer.
Is it worth three grand? With internal 4K, it's hard to argue against that price tag. Yes, there's more that goes into cinematic quality video than just 4K resolution, but the a7sii can produce professional quality video for the web. (And with an external recorder it can produce television broadcast quality.)
