Some good answers here. I'm not usually an IDE kinda guy but I've used both Flex Builder (soon to be Flash Builder) and FDT on projects and I have to admit they can come in handy. Having a debugger can occasionally make development and debugging much easier. Plus Flex Builder 3 has a built in profiler which is pretty useful. I haven't used FDT recently (when I did it didn't have a debugger) but Flex Builder 3 isn't all that "heavy on system" (both are based on Eclipse).
I might add that using a plain-old text editor (e.g. TextMate) can be a good experience. You'll have to learn to deal directly with the compiler or use ant to manage your builds which helps you learn about an important part of the tool-chain. This isn't exactly what you asked for though - so unless you are interested in learning these things an IDE is the faster, simpler option.
In my day-to-day use, if I am writing a 1-2 class project (like an example I will post to stackoverflow) then I use TextMate and build from the command-line using mxmlc. Pretty much everything else is in Flex Builder now (probably to justify the $$ it cost me).
