I've recently been using ANT to compile my Actionscript and Flex applications. ANT is really wonderful when you want to do a little more than just compile. Its really useful for people like myself that have moved from FlexBuilder to FDT4. For starters nothing can compare to the feeling that I have the support and love of the FDT team. Honestly they really make it feel personal but, I digress. The point of that story is that one gets used to certain FlexBuilder features that do not exist in FDT 4. For instance when you compile in FlexBUilder files and folders in the src folder get transfered to the bin-debug folder. There are of course arguments pro and con - but, I do like this feature - ANT gives you the capability to do this, and a lot more...
What is however staggering is the foot print of the compiled swf. I compiled the same application (a full fledged application built with RobotLegs as scaffolding ) with the default Flex SDK debug (Debug button in Eclipse or f11) and with ANT. In practice I usually toggle between F11 and my ANT build script. The default F11 build produced a file size of 983K. The ANT compile created a file of size 76K. Less than a tenth of the size. Mind boggling but true.