Flex is available today for ubiquitous use. A huge amount of documentation, examples, etc.
Flex 2 can run in any browser where Flash Player 9 is installed.
In Flex, you can use any HTTP server and any backend application. (.NET, JAVA, PHP, Ruby, CF, Python) receiving data via XML, SOAP, Sockets, ZLIB, etc.
Flex 2 is a set of expandable components. There is a mass of open source components (in Russian sources) that you can use in your projects. For example, such as - Flexbox, FlexLib, FlexComponents, etc. You can also write your own.
Flex integration with .NET (as with backend technology) is not yet complete. But developers are not sitting still and are working hard on full integration with .NET. Flex 3 (now in beta) supports .NET SOAP
Transferring RealTime data to Flex with Binary Sockets can work with any TCP / IP socket server. FTP / NNTP / SVN / POP / XMPP are supported. For example webmessenger.yahoo.com
Powerful application skinning support, both software and graphics. For skinning you can use Illustrator / Flash / Photoshop / CSS
Many companies are actively using Flex to develop RIA applications. For example, Yahoo and Google.
If you are developing desktop applications using Flex or AJAX, you can use a platform specially developed for this purpose - Apollo (AIR). Using AIR, you can develop cross-platform desktop applications for WIN / LIN / OSX.
Flex3 switched to open source mode using the Mozilla Public License. The benefits are obvious. Now the compiler and framework Flex3 SDK will be available to third-party developers. Those. Now we ourselves can improve and expand Flex.