[MonsterCross]

Don't try it when video is 1080p and display is 1080p or you won't see anything. It's more interesting when playing SD content on a HD display
It's not magic. The kernels are a compromise between sharpening (producing more detail, but increasing noise) and blurring (hides the noise and the detail).
The best option for SD video may be different to the best option for 720p.

It will make a difference for watching a 1080P movie on a 1080P display - but more for the chroma than the luma. So the effect would be a bit subtle. In this case the chroma is being upsampled 2x in x & y.