I used one for a few years. It was a pretty neat thing for its time.

IIRC it didn't work with a CRT using MS operating systems after Windows 2000 due to driver changes or something.

But Timex also a sold little single-LED blinkenbox that plugged into an RS-232 port that would do the trick. That was intended for computers with LCD screens like laptops, but it worked with later operating systems too.