When I am coding a large class file with several nested code blocks such as methods, try/catch statements, if statements, for loops, I have to use Notepad++ to have access to Indentation Guides, because this feature is not built into Flex Builder. Indentation guides are vertical lines in the editor that follow the tab stops in the document so that code blocks can be visually tracked.
To me this is an essential feature: it is a crucial element of code assist, enabling me to keep track of all the open and closed blocks. I wish Flex Builder had this feature, which is found in several other code editors. If you want to see this feature in Flex Builder, please go vote for Feature Request #17634 on Adobe JIRA:
[#FB-17634] Indentation Guides in Flex Builder