Have you ever had a large project in Flash Builder and you need to refactor a whole chunk of code into a new namespace and also have it move the files in SVN?
Have you tried to run an ANT build and got an "OutOfMemory error:PermGen Space" error and needed to up the memory to 1GB or over?
Has your machine been bogged down by massive memory/processor usage?
Much as I like Flash Builder, with its UI features and its ability to publish Flash projects, it has one major flaw: it's built on Eclipse.
Refactoring code and updating SVN in Flash Builder with Subclipse is possible but it can take a few minutes, plus it does have a tendency to go wrong and the whole SVN project can get addled, causing much grief. Doing the same refactoring job in IntelliJ can take seconds and it will also update all your package declarations to match the new namespace. I can link to and run my ANT or Maven builds in a much more pleasing way, and it never seems to run out of memory, and all the while during coding, refactoring or building I'm using < 200MB of memory.
With IntelliJ you can debug swfs running in the FlashPlayer, unit test and refactor with ease. The downside is you don't get the memory profiler, the ability to publish Flash projects, or Flash Builder design view (I'm sure most proficient Flex developers hardly use it anyway).
I've been using IntelliJ 9 for 6 months now (I was previously using FDT), and IntelliJ 10 has added extra support for Actionscript, Flex and AIR. If you develop large scale Flash, Flex or AIR projects, IntelliJ saves time and frustration and is also a great editor for many other languages too.
Here's the Flex features: IntelliJ 10 Flex Features
I still use Flash Builder at home for my own projects. At home I tend to follow Adobe's recommended workflows between Flash Platform applications which all works reasonably well. So it's horses for courses, but for my job which deals with many large Actionscript heavy frameworks, IntelliJ is much more pleasant to use than FDT, FlashDevelop or Flash Builder.