Flex examples and case studies

What are some good examples of flex usage on websites?

none

I'm not sure I understand the question. Are you looking for examples of what you can do with Flex (in which case this is good) or are you looking for high profile sites that use Flex (in which case the Flex homepage mentions some). Or are you asking about site that use Flash well (its far harder to know if they use Flex well without having their source)?

yes I am looking for websites built with Flex

Then Adobe's Flex home page (linked above) is a good place to start.