I've seen these kinds of discussions many many times. They usually start with WHICH Flex framework do you use. Not many people ask the question WHY do you even need to use any framework on top of Flex framework.
I'm not in favor of using any MVC framework (Cairngorm, PureMVC) in Flex code. Mate is a better candidate. At least it's simple to understand and is non intrusive. I prefer using enhanced components
à la carte. We've created and open sourced a bunch of them (see clear.swc in the Clear Toolkit at http://sourceforge.net/projects/cleartoolkit/.
The first chapter of our upcoming O'Reilly book "Enterprise Development with Flex" has a detailed comparison of several Flex frameworks: http://my.safaribooksonline.com/9780596801465 .
