I have an Integral knock meter in my NA, and an HKS knock monitor set away for my turbo build.
The HKS one isn't sold by HKS-USA, since it does not meet FCC regulations.
What is unique about it is that it has it's own sensor that you install in the engine bay.
You can set an alarm point where it will give a visual, and audio alarm.
It has an ear piece jack, so you can plug in headphones.
You can switch between just knock detection, to actual engine bay sound.
And.......
It will transmit over any nearby FM radio set at something like 88.5 (the reason the FCC doesn't like them).
If you want one, you need to get it from someone who imports parts directly from Japan.