My Halo 3 run incredibly faster now- although it isn't really mine.
My Halo 3 disc got really scratched somehow, so I borrowed a friend's copy and installed it to my 360's HD. Now, with the image mounted up, load times are much faster, because it doesn't have to sort through scratched sections of the disc. Before, some levels weren't even playable, and now they are, all because I can install a game and only use the disc to prove I own the game.
This feature will be very, very useful for people who have scratched discs who also have friends who can lend them a copy.
Rip friends' disc -> play a previously unplayable disc.