If you're doing any kind of serious development in Flex, you need to be looking at your test cases and the code coverage you're getting.
One of the challenges for any kind of incremental development, especially the kind of rapid prototyping efforts that Flex and FlexBuilder are so naturally suited for, is keeping an eye on where you need better testing - both automated via unit tests but also manual.
Hence my delight at finding the flexcover tool recently.
Not only is it now dead-simple to install (since version 0.5 came out), but you can use it in a fully interactive mode where it shows *in real time* the coverage of your code during any kind of application usage.
I've been letting it run in the background while I manually test parts of my evolving application to give me an idea of what needs attention, what code paths are dead and ripe for pruning, and generally, how much confidence I can have in my manual testing vs. the automated units
Check it out.