Hmm. I don't know about how much they leak, but as the listener I find that they are better at isolating/blocking out outside sounds than, say, the Beyers or Senns I have. (I know however this is not the concern of the original post-- blocking the sound for others around the listener).
Is there an identifiable closed can as opposed to open can sound? I need more experience with closed cans, as the Denon's do not appear to be properly representitive.