For developers having worked on J2EE web applications for many years, getting into Flex will seem both very fun and familiar of the simplicity and power of ActionScript and the UI framework, and quite tedious and frustrating when it comes to developing the core application logic and the server integration.
In some ways, developing Flex applications with widely used frameworks like Cairngorm and BlazeDS involves a lot of plumbing code (business delegates, service facades, conversions between JPA entities and value objects,...) and will remind you of the days of Struts and EJB2.