There was also a lot of value in filling out the application. I had my cofounders write their own version of the application, so that they internalized some of the values -- they even came up with several ideas while doing so (and merging wasn't so difficult as I feared). We've written and rewritten our pitch and our mission several times now, and it's starting to get very good. It was definitely helped by the need to put it into YC's format -- now we have a punchy answer written for all of the common questions.
However, we think that YC is one of the best deals out there. There's the networking of course, but then there's the PR, or the 'social capital'. If we impress PG enough, he'll say something shiny about us in an essay, and suddenly our website grabs the starry eyes of thousands of readers. It's also probably worth a lot in confidence to be picked as a probable winner out of an application pool of more than a thousand.