You don't need FlexBuilder2 to develop FDS apps. All you need
is FDS Express and the Flex2 SDK. That said, FlexBuilder2 makes for
a far more pleasant ActionScript3 and MXML coding experience and I
find the integrated debugger to be priceless. I run FlexBuilder as
an Eclipse plugin and it's great to be able to step through client
code and Java server code at the same time.