I don't have HD TV yet
However, my m8s have. One has a 26" Bravia and the other has a 40" 5th Gen Samsung. On either the HD visuals of the X360 are stunning.
You can read the text on game. One screen maps (like the one in GTA IV) become crystal clear.
It's like playing a game on your PC at 640x480 res on a CRT then getting a nice spangly 24" widescreen LCD monitor and playing the same games at 1920x1080.
Now it makes sense