Visual Studio is really popular with developers and for good reasons. It's fast, has some of the best Intellisense and debuging there is and now you can also use it for Flex authoring. Amethyst is developed by SapphireSteel and it's currently in public Beta.
One of the smartest moves Adobe ever did was to release the Flex SDK and make it easy to integrate it with other code editors. We know of quite a few editors (commercial and open source) that use the SDK, so this is certainly extending the Flex echo-system. Amethyst is a free IDE that builds on Visual Studio. If you already have Visual Studio and the Flex SDK installed, you only need to run the Amethyst installer to get started.
Amethyst offers code completion, formatting and intellisense for AS3 and MXML. There's class templates for AS3, AIR, MXML, MXML components and more. The code is color coded and collapsible and all the neat Visual Basic panels such as Object and Solution Explorer works. The Personal Edition of Amethyst will be a free download for always, but some IntelliSense features may be provided in the ‘Professional’ edition only. The latest version was released only days ago and the beta now has super-fast intellisense, so if the slow intellisense in Flex Builder is killing your productivity, this could be worth a shot.
Check out more and download from here