I have been using Flex3 and try to switch to Flex4 but I have been very disappointed with the performance. I was hoping Flex4 would improve performance. I have also been amazed how few Flex3 or Flex4 websites are out there. This the FlagShip design approach of CS5. But the reality, I am not sure it was ready and seem the market is agreeing. I have seen the Flex4 websites that pushed as examples, But I have not seen one real commercial website. In Fact Adobe used Flex3 at one time for Adobe TV. But no more.
I would love to see any real sites you may no about.
I would like to see Adobe add the ability to export to html5 code from Illustrator until Flex4 is more complete and much faster.
One other major concern is my test do not show Google able to read Flex4 swf files. Big issue!