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 -_-
As of PHP 5.5, the MySQL library has been officially deprecated. It is recommended to move to either MySQLi or PDO libraries for your mysql connectivity. See here for help choosing which interface you prefer: http://php.net/manual/en/mysqlinfo.api.choosing.php