Step by step debugging with variable view (and often with additional views like stack or global variables etc) is working in some IDEs. FlexBuilder, FDT3 Enterprise (only the enterprise version allows debugging), FlashDevelop (with extra plug-in). You can also try the (free!) Visual Studio plug-in Tofino (Ensemble), so you can debug inside Visual Studio.
But the best implementation for debugging I found in the Eclipse based IDEs. I'm not much impressed with FDT3 at all, so I recommend FlexBuilder for debugging (+profiling) and of course coding of AS and eventually (visual) design for MXML.
You can try FlashDevelop with the FDB plug-in. Many people prefer this free IDE for coding because of the nice code assist (IMHO the best of all known IDEs). Debugging is working with this plug-in but I wouldn't say that is the "most productive". But give it a try, its free!
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…