As a former Flex developer, I must admit that Flex was the best thing that ever happened to web development, but unfortunately the Flash team wasn't strong enough to bring the runtime to mobile with enough quality. Building a RIA in Flex was so incredibly easy, and debugging was easy, and you really could use anything you wanted for a backend (I've used Perl, PHP, and Java). Bidirectional data binding, an excellent widget library, and a lot of help from the default IDE (custom build of Eclipse) allowed me to build software, full stack, with an engineering team of 1 on a weekly release cycle.
I've spent more time debugging HTML5/JS/CSS than building entire applications in Flex. The AdvancedDataGrid component alone was worth the price of admission.