I agree with McKee that using V.O for the most part, is horrible and should be avoided. But the point is, a REAL screenwriter should know all the rules... so that the screenwriter can break the rules to write a stunning script. Such as with Dexter.

They also use a ton of Voiceover in ADAPTATION.

Think about it :)

most voiceover sucks because it is telling us something that is either pointless, obvious or best actually told.

on Dexter we are hearing things that can't be told outloud. they are almost the voice of the DarkPassenger. which is why it works.

another good example is Shawshank Redemption which uses VO as a way to bridge chunks of time.