I have routinely and successfully used metaphor in teaching mathematics especially when describing how the brain works. Every once in a while I find a student who resists: "Why are you talking about learning to keep your balance on a bicycle? I thought we were talking about studying for the chapter 5 test?"
I found the following video, newly added to the TED.com repertoire. James Geary is amusing as he describes the importance of metaphor in human thought.