I have, and I do not like it.
For a designer or front end developer, its not bad - makes nice interfaces and easy to use; it is also quite light weight considering its origins.
From a backend point of view (where my programming originates), it is quite difficult to get to do the most simplistic tasks (comparisons for examples, I just made a post about it here on the forums), while some of the more complicated tasks are easier (like trees).
All and all, if you are a designer and don't care much about programming, flex is a good choice. If you are a backend programmer, you will likely prefer using a more robust language for your application, let it be PHP/PERL for web based, or C/C++, Java, C#, VB etc for desktop applications. Fortunately, FLEX does have support for both web based and desktop based applications, so that makes backend support much easier. I do not recommend using actionscript as an actual backend language, it is not too pleasant to work with -_-
header('HTTP/1.1 420 Enhance Your Calm');