I own a copy and I use the free Flash Develop. It's not any one thing in particular, it's just that it has inherited that big and Enterprisey clutter from Eclipse without any of the powerful tools (the exception is the profiler, which is very good and the reason why I own a copy, but that only comes out for a day at the end of a game development cycle). I'd rather command line compile than use Flex Builder for day to day tasks.
