Why are USM II super teles so expensive?

I was wondering if anyone has any experience with the USM and USM II super telephotos who can tell me whether the incredible mark-up of the II versus the I, or over their Nikon counterparts, for that matter, is worth it. On Adorama's website, for example, I see that the Canon 400 2.8 USM II costs $11,500, whereas the Nikon equivalent is "only" $9,000. I know with these lenses you usually get what you pay for, but are you really getting 25% more with the USM II lens?