Very good camera, I have had it for 2 months and I use it with the sigma 18-50 F2.8 lens. I have taken several photos, videos, and timelapses and I am fully satisfied.
In summary, we have:

Positives:
- Very good and clear photos with room for cropping in editing if needed, thanks to the 24MP.
- Excellent 4K video and 1080P slow motion.
- Sturdy compact construction made of magnesium and weatherproof (the only thing that slightly concerns me is the two plastic doors, one for the battery and the other for accessing the ports, but if you don't force them, you won't have a problem).
- Very fast and good autofocus that recognizes faces and eyes.
- It has a port for a microphone.
- Large menu that you can customize to your liking.

Negatives:
- Relatively short battery life (personally, I didn't have a big problem as it charges quite quickly and with a power bank, you can charge it while using it).
- The grip is not the most convenient due to the compact design.
- The screen is touch only for focusing and not for navigating the menu.
- Due to the large menu, it takes some time to get used to it.

In conclusion, I think it is an ideal camera for someone who is interested in photography and video, both as a hobby and professionally

I mainly use it for videos but whenever I need it for photos as well. In the photography department, it is very good, its auto focus is top-notch, it has satisfactory colors, fast shutter, lightweight, and it never tires you with its weight.
Now, in the video department: initially, you have a lot of flexibility in frame rates since it even gives you 120fps in 1080p, s-log and hlg are very convenient for color editing in post-production, the sensor with the appropriate lens produces a very nice bokeh for more cinematic shots but! The fact that it doesn't have built-in stabilization makes it unsuitable (e.g. for handheld video if the lens doesn't have stabilization, you definitely need a monopod or some other external stabilizer), its menu is not so user-friendly if you want to quickly change frame rates on the camera, and the most significant problem this camera has, which also significantly affects the photography aspect, is the battery! For a wedding, I needed at least four batteries, which I find very tiring and expensive...
Overall, it is a good camera without any major limitations and with satisfactory quality. Personally, though, I would recommend going for the a6600, which has a much better battery, built-in stabilization, and essentially the money you would spend on extra batteries comes out to be about the same price as the a6600. Thank you for your time.

I have been using the a6400 for about 7 years for video as a second camera, backup, and for shots with a gimbal. If you want a small body with professional capabilities, I highly recommend it. At 950 euros with the kit lens, I think it has no rival. I chose the a6400 mainly for its unlimited video recording, low weight combined with a gimbal, and of course for its fast focus that will never let you down. The kit lens is fine for the money for amateurs but has limited capabilities. If you want to do everything with one lens, especially in video, then the 18-105 F4 with power zoom is the best choice for this body and will serve you incredibly well. Of course, there are more expensive lenses that greatly enhance the quality of the results with better brightness, such as the Sigma 16mm f1.4 but with attention to focus, the Samyang 12mm f2 autofocus combined with a gimbal, and the Sony 35mm f1.8. I believe it is a body that you will never sell.

POSITIVES: Low weight, incredible autofocus, unlimited video recording time in 1080 without overheating and quite good time in 4K. Touchscreen with continuous AF and tracking focus. Screen adjustment in sunlight. The lack of stabilizer in the body is not an issue when there is a stabilizer in the lens. I would like a larger viewfinder, but even with this, you can get your job done. It works with a dummy battery.

NEGATIVES: Relatively cumbersome menu that you get used to over time; if you have been a Sony owner for years, you won't have any problems. Short battery life (which you can solve by using a dummy battery with a power bank), at high ISO the video image breaks, max 2500 ISO. Lack of stabilizer in the body. It doesn't have a good grip, a problem that can be solved with a cage.

At 1099, along with the 18-135 lens, it is the best value for money in APS-C. The lens alone costs around 600 euros and the body around 800 euros.

The major "issues" with this body are two.

The first, and my biggest fear before purchasing, is the poor rolling shutter in 4k. I worried for no reason. Of course, the problem exists, but in order to be noticeable, you either have to use a lens without stabilization (the 18-135 has it), or intentionally make very abrupt movements. Most videos on YouTube that emphasize this particular problem are shot with the non-stabilized (but otherwise excellent) Sigma 16mm f1.4.

The second problem is the not-so-sharp 1080p video. Indeed, cameras like the Fuji XT-3, XT-4, XT-30 have better 1080p video. However, they also have time limits for recording, which are due to overheating issues. The a6400 has no time limit and never overheats, neither in 1080p nor in 4k.

For stills, there are no comments, it simply excels. The same applies, of course, to all Sony APS-C cameras, from the a5100(!) to the a6600.

Easily 5 stars!

After years with the a5000 and the classic 16-50 kit lens, I upgraded to the a6400 with the 18-135 lens.
The menu didn't seem difficult at all, as it's pretty much the same on all Sony cameras and I didn't have to learn it from scratch. Personally, I find it very simple and easy.
The custom menu also helps, as one can create their own settings, and there are also many custom external buttons that can be configured to control it even faster.
I won't go into details about the capabilities, as there are many and I would become tedious or surely miss something. There are dozens of websites and videos on YouTube that explain in detail what the camera can do and the differences it has from other models and competitors. I will only mention the main reasons why I chose this one instead of the a6500 that I initially considered:
The biggest reason is that there is no limitation on video duration. All other Sony cameras have limitations due to overheating. Some can record for 20 minutes and then shut down, others for 30 minutes. The Sony a5000 I had in the summer could record for a maximum of 35-40 minutes before shutting down due to overheating, while in winter I managed to record up to 1 hour and 15 minutes.
The a6400 has solved this major drawback, and theoretically one can record video until the memory is full or the battery runs out.
Now let's talk about the other major problem with Sony cameras, which is the battery. The a6xxx series has been using the same battery (NP-FW50) for years, which in some models (like this one) can only last for 350-400 photos or about 1 hour of video. The cherry on top is that almost all cameras in the a6xxx series do NOT support camera operation while charging. You either have to use it or charge it.
The a6400 now supports simultaneous charging, so this chronic battery problem is solved. If someone wants to shoot a 2-hour video, they can connect it to the charger or a good power bank!
This particular camera is almost the same as the a6500, but without the sensor's built-in image stabilization in the camera body, so it is considerably cheaper.
However, if someone plans to use lenses with their own built-in optical stabilization, the result is exactly the same without having to buy the much more expensive a6500!
In terms of other details, the screen is quite good, but ultimately not as bright as I expected under the sun. This is where the electronic viewfinder helps a lot, as it is much brighter than the screen and allows you to see clearly what you are aiming for, no matter how bright the day is.
Some details that bother many people who shoot videos of themselves, but not those who take photos or normal videos, is that if you connect a flash or microphone to the hot shoe and rotate the screen 180 degrees forward, the microphone obstructs the view of the screen.
Personally, it doesn't bother me at all, but for those who have such an issue, they can easily solve it by purchasing a "Hot Shoe Relocation Plate" for 10 euros and do the job.
Regarding video, the only thing I would like it to have, and I will probably miss, is a headphone output to monitor the sound in real-time.
The 18-135 lens is light years better than the standard 16-50 that Sony usually includes.
It focuses faster, has better colors, better sharpness, and is also quieter when focusing! The zoom is manual, but still quieter than the 16-50 lens, while the photos have excellent sharpness even with the aperture fully open at f/3.5! In comparison, with the 16-50 kit lens, you have to go above f/7-f/8 to start getting the same level of clarity, and that's only in the center! The 18-135, closed without zoom at 18mm, has a slight distortion (I think all zoom lenses have a little bit), but definitely smaller than the kit lens, and chromatic aberration exists but to a much lesser degree. In low light/night shots, the lens is just as dark as the basic one since both start at f/3.5, the only thing that improves things is the sensor of the a6400, which can handle high ISO without much noise, especially in videos where it can easily reach ISO 6400-8000 and have minimal noise that can be easily removed with some editing later on! I tried it with a cheap Chinese lens, the 35mm f/1.7 for 45 euros, and it was like the camera gained night vision! I know that the price difference is significant, but judging the classic standard lens 16-50 against the 18-135, I would say it's a miracle that Sony managed to market the entire a6xxx series based on such a cheap and poorly made lens! In conclusion, I would like to mention that personally, I believe that the combination of the a6400 with the 18-135 lens is one of the best combinations available, as they will cover 95% of the needs of someone who enjoys photography (or videography) but doesn't plan on buying many different lenses soon, as with this lens, they can capture wide-angle landscapes and portraits with enough bokeh, as well as distant details in landscapes or faces.