iZed,
Welcome aboard.
I take as much of my own stuff as possible. My local store showed me a £1.5k projector linked up to a £2,500 dvd player! I wasn't happy with that and took my own one in.
I have a CD wallet thing I stick a load of discs into. The ones I currently use are:-
LOTR - Light hobitron scenes. Loads of grass and intricate detail and scene panning. Dark scenes to see blacks and check I can see the shadowy detail.
Ice Age - Loads of whites and solid colours. I'm looking for banding in the solid colours and checking whites look natural.
The man who wasn't there - New film in B&W. Make sure B&W is displayed correctly. This disc has shown up faults I didn't see when viewing the other discs.
SWII - When Anakin kills all the tuskan raiders - very dark scene see how much detail I can see.
Life of Brian - Old print quite grainy.
I have a GC and Xbox. I didn't bother taking those in. I guess it's nice to see it works but I've never really seen a PQ problem playing a game. Much easier to spot with a few choice film demos.
I'm also carrying cd's and audio discs for audio demos....
Morcheeba (their first album)- Light clear female voice.
Chemical Bros (Surrender) - Dance music
Marylyn Maison (Coma) - heavy rock.
Police Greatest hits DTS
Metallica DVD-A
I haven't found a satelitte system that can handle all of those sufficiently yet
Never used to like demoing. Love it now

Jus.