To be fair, it's more than a test card. It's a program that enables you to set up your TV to optimise your HD picture. It also can show the limitations of some TV's in how much they 'overscan' and how much of the picture falls off the edge of the screen.
I did try setting my TV up using this program, but personally found the results calibrating with the DVE DVD a better option. Despite this being via my DVD input, admittedly my DVD is connected via HDMI too, so that may be a factor. For anyone who has the Sky box as their only device connected by HDMI it's well worth having a go with Mylene!