Flex 1.5 introduces charts, RSL and SWC compilation support, HorizontalList and TileList controls, and many new enhancements on top of Flex 1.0. Applications built with Flex 1.5 work with Flash Player 7 or above. Read the Release Notes, Installation Instructions and enhanced API documentation.
Flex Builder 1.5
Flex Builder 1.5 is the version of Flex Builder to use for Flex 1.5 projects.
You can have Flex Builder 1.0 and 1.5 installed on your machine at the same time. The same serial number will work for both, so if you have a serial number for Flex Builder 1.5 or 1.0, you should not have to enter a serial number again. If you are using the 60-day trial of Flex Builder, both versions can be used for that trial period.
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Source : http://www.adobe.com/