The case against Flex-based application UIs: Love this post from Tony Byrne. I agree with every word of it.
To me, turning to Flex for a content management interface is a cop-out. It creates nice demoware for the vendor, but long-term problems for you. I can see why Flex is alluring for vendors: maintaining consistent, cross-browser compatibility (especially with AJAX) is hard and expensive. But why does that mean that you the customer must give up the simplicity and supportability of a native browser-based interface?