Ok, so I've been looking into Flex these past few weeks. From a quick glance, it looked promising: XML markup to declare components, the ability to extend component via ActionScript 3, and a nice data binding scheme.
While I respect the efforts the Flex team made into creating a nice application development platform, I found that using it was far more restricting that I would like, particularly when it came to styling applications.
The CSS support that Flex provides is horrible, to put it nicely. It seems that the Flex development team never ever built a single CSS-based layout, or any post-1995 layout for that matter.
The official way of styling text (directly from the docs) involves having 2 nested nodes, with <FONT> tags in a CDATA section to define basic things like font size.
Flex does allow us to put some styles in a separate stylesheet file, but I wouldn't call it CSS per se. It provides mechanisms that are not part of W3C's specs, and it fails miserably at rendering styles from something as simple as an id selector. Class selectors with underscores even throw compiler errors!
Not only that, basic rules like left, top are not supported in CSS (they need to be inline in the MXML file). Even more basic things like text-align are not supported at all in some components! The closest attempt at fixing it (that I could find) is a huge 30 line monster. There must be some really dumb aspect that I am overlooking; it really shouldn't have to be that complicated.
One other thing worth mentioning is that the extra compiling cycle can really slow down production, especially when running into so many CSS support issues. A fairly simple application takes on average 5 seconds to compile. Considering also the learning curve associated with any OOP framework, 5 seconds of compile time is definitely not to be taken lightly.