Flex should be your choice only when your inhouse team is not good enough at churning out sweet OOP Actionscript. This is because, using Flex makes your app depend too much on Adobe's components. This is not required in most cases. Writing custom components from scratch is the best option[making Flash IDE your primary choice].Flex should be a secondary option.

[Flex was created to make Flash Platform more accessible to Java developers who are not familiar with the timeline concept]