Dean Wampler explains here.
I have no faith in using tools for drawing and generating code. It sounds nice, I just don’t think it’s a good solution for most systems. A friend of mine has been working with MDA for a while, and he claims it works ok for boilerplate basic webapplications with a few forms etc. It might save you some time on these, but I’m still convinced that sooner or later you have to get into the code and then you have a maintenance nightmare on your hands.