Metaphors are best when they clarify information. Referential metaphors can provide a cognitive pathway to understanding by comparatively turning technical subjects into more common subjects. However, you can overuse metaphors if they don't clarify the point you're trying to make.
A common mistake in using metaphors is mixing them, thus confusing the message. Something like, "Kill two birds with one bush" mixes the common metaphor of "Kill two birds with one stone" and "A bird in the hand is worth two in the bush."
Metaphors can have the power of not only elucidating one meaning through comparison, but also multiple layers of meaning. Deeper metaphors can touch on powerful subjects, like human nature and the meaning of life.
According to the website ABC Copywriting, one example of an effective metaphor comes from an insurance company in Britain. "Leaky Umbrella" is a phrase the company used to market their insurance. It was used to emphasize other insurance companies' poor coverage, just like a leaky umbrella provides little protection to its holder.