It is a frustrating camera - outstanding in some aspects, most notably the AF system, good in others, but mediocre in some key areas by today's high standards. Overall, it's still a good bet, especially if you shoot a lot of moving subjects, but it also could potentially have been so much better...
It is a near-universally capable midrange camera that offers some of the best autofocus performance we've ever seen, backed up by solid image and video quality in most situations. We'd really like to see some further refinement on ergonomics, as Sony has done with its full-frame models over the years, but the a6400 remains an excellent option for a variety of types of photography.
The compact size of the Sony Alpha A6400 makes it a great choice for travel compared to a larger full-frame camera. Image quality is impressive, with the camera delivering great image quality, with improved noise performance. [...] The menu system takes some getting used to.
It s a great evolution of the a6300, and is a more complete camera in every facet. It has significantly better autofocus, an improved physical design (complete with “selfie-screen”), and has smoothed out a lot of the rough edges (overheating, anyone?) For the moment the upgrade path is muddied by the existence of the a6500, which has some superior features (IBIS, buffer) combined with a few inferior features (focus system, silent burst rate).