Anybody concerned about embedded malware spyware etc? HDMI can carry ethernet so I'd worry this might "phone home" or even take over an attached computer.

I bought one of these a while back as a curiosity, I tried it with my MiSTer and while it worked it was a little bit laggy for games with a bit of screen tearing.

Saying that, it has proved to be useful for work, I've been able to use it to write documentation and capture screenshots and videos of things like BIOS and UEFI settings screens and as mentioned by another commentator it's handy for headless systems not having to have a monitor kicking around.  For the price you can't really go wrong and with it's small size it's easy enough to keep in a laptop case or tool kit.

This saved lot of my time. I was building my Lenovo mini pc NAS server then I used this to connect the mink pc screen out to my laptop, So I could use it without having extra monitor, and then after setup I did SSH.
Thanks