I've been building apps since the first beta of Flex 1.0 and most of the time people seem to pick Cairngorm or some variant of it for their apps. We're currently using a lot of the concepts from PureMVC in our refactoring effort for a large Cairngorm app. Dropping a new framework into an existing app (especially a large one) can be a non-trivial task. We've opted for a more incremental approach to updating our app framework. The only downside to this is that we end up with a custom framework.

The primary reason for the change was an attempt to eliminate some of the boilerplate code that Cairngorm requires. This wasn't necessarily a problem for the more experienced team members, but the ones newer to Flex felt it was just too hard to understand and found writing boilerplate code too distasteful. So now we have a framework that is less like Cairngorm, but with it's own set of issues.

Choose wisely grasshopper, lest your future struggles be more "interesting" than those you are trying to escape.

That said, my personal preference for Flex Frameworks is Mate. I find it to be very clean to write apps with and i really like the tag framework style that it and Degrafa provide. I'm using it for all my component development testbeds these days...