Seems like voice over works best when it gives you the point-of-view of the character, esp. a character that would be hard to sympathize with. The narrative lets you see things from their perspective. This is notable in A Clockwork Orange, Goodfellas, and Taxi Driver. These movies would be nothing without their narrator. The characters would be distant, unrelatable, violent monsters. The narration lets us understand what makes them tick, and see the world the way they see it. Then again, I'm one of those people who has no problem with Harrison Ford's narration in Blade Runner.

(This is going to come across as annoyingly didactic - sorry, it's what I believe...) I think it comes into the same bracket as subtext; the off-screen screenplay; starting the scene as late as possible, and leaving as soon as possible; letting the actor turn a line of dialogue into a gesture. They all have one thing in common: the audience has to fill in the gaps. Which is what makes them a participant instead of a spectator. A well-told story doesn't have spectators, it has participants. It's a balance of what to show and what to leave as blanks to be coloured-in by the audience. So voice-over can be evaluated on the basis: Does it enhance that balance, or does it screw it up? Theoretically that implies you could take a script that "needs" voice-over and provide one that fixes what's on the screen effectively. Of course that almost never happens. It also suggests a reason why the Bladerunner voice-over is not universally condemned (I'm happy both with and without) because it doesn't upset the balance.

Like anything it has to be done well. I love it in Stand By Me. I can't imagine the movie without it. One way to get out of a writing jam is to start using voce over when you're lost - usually when you do this the voice over stuff you write can be turned into action instead. For some reason writing it as voice over first liberates the imagination because you're suddenly juxtaposing the character against what should be their journey. Sometimes voiceover is not there to explain the plot. It's a way to fit a bigger story into a tighter time frame. Just think about the voice over/direct to camera exposition in Avatar. Talk about crafty! I think anytime you stumble on arbitrary rules you can go walking in the creative desert for lack of direction. And I've spent lots of time doing that walk. It's better not to think about it. The devil truly is in the details. Don't let the devil mislead you and separate you from the joy of the work. Because that's what devils do.

When the the voice over is trying to fill in the gaps in what is otherwise weak storytelling, it always falls short. I think it works best when it reveals the inner world of a character, letting the audience get to know that character on a deeper level.

I have used voice over when I needed to hear what a character was writing. It was the most efficient way to get the information across.