For my Flex project at work, I use Cairgorm as the MVC framework. Not that I like Cairgorn a lot but at the time our project starts it seems to be the de-facto framework for mid-to-big-mess Flex applications. Adobe also stands behind Cairgorm in supporting and consulting it.
The learning curve is not so steep. I pick up the tutorial material available at Adobe website, specially the animated diagram is very helpful and learn to use the framework in less than a day. The second day I already begin to integrate it into our application.
One thing annoys me most about this framework is that there is no way (sort of) for the UI to get notified when a remote call is done getting data and the result is available. I end up use some hack to get around it. For example, use a ChangeWatcher to monitor the model. When the result ready, the command object updates the model, just triggers the ChangeWatcher’s propertyChange event.