I don't feel like I'm very good at metaphor, so I don't have anything specific to say about it. I'm also trying to think of a specific one that stuck in my head, but I can't, which just goes to show you! But I do know that I love being made to do a 180 by a well-timed metaphor or simile--and not in a punny, cute, Pratchett-y way. I love being surprised by an apparently contradictory metaphor. Maybe it's the irony.