I agree with netherspirit. But... this card can get your opponent by the balls if the following conditions are met:
1) You're not playing "real T1" (with moxen and that kind of stuff) and mainly walk in an unpowered metagame.
2) You're playing a deck that has a long-term gameplan (typically control). That is quite important, because if you
play a speedy Deck, your opponent would never had drawn the removed copies anyway: In that case, you don't give
your opponent enough time to dig through his library. Duress or Cabal Therapy are better choices.
Since (real) T1 IS typically the speediest environment, Duress or Cabal Therapy are (lightyears) better than Cranial Extraction.
3) Your opponent plays a deck, that relies heavily on only one 4-of-Card. Take Oath for example. If you manage to counter the
first Oath and then aim a Cranial Extraction at the remaining Oath-Copies: GG.
I personally like to side in Cranial Extraction against opposing Control-Decks. Aim them at their FoW or Drains, and they are forced to
counter it. If they do/can, you had drawn a Counter. If they do/can not, the opposing Deck lacks a serious amount of Control.
For me, Cranial Extraction is only a sideboard-option. In the first Game, you often do not know wich card to call, because you don't know
the deck you're fighting against.
Over and above Cranial Extraction, I'm also a big fan of a mid-game, Burning-Wish-fetched, drain-powered Haunting Echoes. Let the opposing
graveyard contain only 4 to 5 "4-of-cards", and the game should be your's. But that would be another card discussion
