Flex, which runs on top of Flash, is a very good platform for building applications. In my opinion, this is much better than trying to get HTML and JavaScript to be a platform for GUIs. In addition, if you have a graphic designer, it will be much easier for them to design their appearance using the WYSIWYG tools available in Adobe tools.
However, for traditional websites, I adhere to HTML / CSS / JavaScript. Do not use Flash if you need simple effects.
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