This past Monday, Nathan Phelps gave an excellent presentation on Adobe Flex 2.0 for the Tulsa Java Developers Group. I was totally amazed out how similiar it is and fundamentally like Silverlight (formerly code named Windows Presentation Foundation/Everywhere (WPF/E). I see the pros and cons from both architectures. Once Silverlight is fully released and is truely cross-platform as it is planned, then in my opinion it will have many benefits that will surpass Flex. Flex has the power of Flash transformations and can be utilized on lower end graphic cards. Silverlight has a subset of the .NET CLR running on the client side, allowing things like asychronous webservice calls, whereas Flex's webservice calls are sychronous. When the whole goal of these new application frameworks is responsive and immersive user interfaces that seems like a major drawback.
XAML and Flex's MXML markup languages have a lot of similiarities though. Now that Flex's price entry point has been reduced through their Open Source offerings it does make it more of a competitive technology, but the .NET CLR capabilties cannot be beat.