I've recently started working with Cliff Hall's PureMVC ActionScript framework (an alternative to Cairngorm or ARP for example) and after investing in a day's training and some sit-down-and-read-the-docs time I really started to like it. I believe it fills a gap in the market for coders like myself who know their way around AS, Flash and Flex but who struggle sometimes with implementing a good architecture. And that's where frameworks such as PureMVC come in - they force you into a best practices approach and to me that's exactly what I needed. You end up with an application that is much tighter, better structured and scalable, yet easy to maintain.
PureMVC is a bit of a new kid on the block when it comes to MVC frameworks so it's great to see that it stacks up well against the alternatives, so well in fact that it came out on top during a shootout at the Silicon Valley Flex Users Group (SilvaFUG).
PureMVC is my chosen framework for the time being and I am using for all my current projects.