The benchmarking data is now live and you can see it if you go to Visitors > Benchmarking (Beta). Google compares the following values for the last 30 days: visits, page views, pages/visit, bounce rate, average time on site and new visits. By default, your site is compared with other sites of similar size, but you can restrict the benchmark to general categories like: Internet, Travel, Shopping, Reference etc. Since the data is aggregated from the sites that agreed to participate in the program, it may not be representative. Google says it will add new categories once more sites will enable the data sharing option.
"When benchmarking is enabled, Google crawls the websites in the account then categorizes them by vertical and the amount of visits. The data is then made anonymous through aggregation. For sites of a similar size, a category of industry verticals can be chosen when there is a sufficient number of accounts in that category."
It's interesting to compare your site's traffic with these aggregate data as it will help you put things in perspective, but you shouldn't be disappointed if the comparison is not favorable. Each site is unique and has a different raison d'être.