Halo 2 Vista project lead Jo Clowes says it only takes about two minutes to start playing from the moment a user inserts the Halo 2 DVD into his PC. While the user plays, Tray and Play actually installs the game in the background—without adversely affecting performance, according to Microsoft's internal testing. Users can check on the installation process by going in the game's main menu and checking on a progress bar, and they can even quit the game and let it resume the installation next time they play.
That said, Tray and Play isn't a substitute for installing the game altogether: the technology doesn't actually support playing from the game DVD alone. Nonetheless, this feature definitely sounds like it would be a good addition to PC games, provided Microsoft licenses it (or other studios end up copying it.)