By Kujata.Akeda 2009-09-11 21:38:17
It asks the question: if you could have a 7 card deck, which 7 cards would you choose? Really what you are doing is trading your entire library in return for 7 specific cards.
Use it with Relic of Progenitus. Use Relic to RFG anything you want, and then when you sac the Mirror to use it's ability, put Relic's tap ability on top of it in the stack. Then grab Mirror itself as one of your 7 cards, and you can tutor yourself any 7 cards that were in your library when you first activated it.
You could also use it with Beacon of Tomorrows and have infinite turns or even as a last ditch effort to win a loosing game by getting back your most powerful cards or making some combo.
FWIW if you pick the wrong cards you'd automatically lose because you have no cards in your library to draw.