I'm not using the Flex framework for this project, just compiling standard Actionscript. Despite its name, Flex Builder is not necessarily for builing applications on the Flex framework. Adobe is renaming the next version Flash Builder to make this a bit more obvious. I have used the Flex framework profesionally before, and found it to be extremely useful but occasionally a time sink.
I've been using FB since shortly after it was launched. I'm a huge fan. It has a lot of features that really speed you up once you get used to using them. As a long time Flash developer I'd always relied on trace statements and observation to debug problem. Having a real debugger (that actually works) was like night and day on its own. To be honest, every time I'm forced to work in Flash CS3 or CS4 instead of Flex Builder I find it very painful now.
There are some other options out there; FDT and FlashDevelop, but I prefer Flex Builder's official support.