I hate Flash. Any day I would suggest to use AJAX to give your web application that extra pep.
Having said that, lets look at what Flex could do for you. Flex is flashy, Flex is fast to develop. Above all, Adobe has come up with a decent framework for application development.
1. Flex is a complete SDK. Flex is a combination of Action Script (an EMEA Script supporting OOPS) and MXML (XML based UI definition). That good news! But then, they combine to produce SWF files and use the much hated Flash Player to render.
2. A complete IDE in FlexBuilder. Based on Eclipse, this wouldnt let you down.
3. Integration to call REST, SOAP web services. Making REST calls and modifying dom on the fly is a must for any web 2.0 framework. But SOAP? Looks like Adobe is serious about the whole Flex business.
4. Localization support using Resource Bundles. Remember SVG and the NLS support pains? Flex is meant for serious application development.
If you want to develop jazzy looking web 2.0 components for your application in a short time, Flex is worth your time. Remember to use it very judiciously.