Apparently they are trying to circumvent the HDCP compliance requirement of "no easy end-user access to decoded analogue video" by making it a dongle. I can't see how that could legally stand up, because all you would need to make all the (analogue) HD copies you might want, is a cable adapter.
Technically the concept itself seems plausible, I have no direct experience with this (or any other) similar product. But of course, to get any real benefit out of this, you need to have a HD display device - otherwise, all you get is a picture downscaled to SD, and you could probably get the same end result with a direct analogue connection.