So what is this neat, new technological marvel going to be used for? Why, to kill, or threaten to kill, American supercarriers operating within a couple thousand miles of the Chinese coast line, of course. Why would they ever want to do a thing like that? The Chinese have some very specific interests within their regional sphere of influence. These interests might be frustrated by the presence of U.S. combat forces in the neighborhood. Creating the ability to deny the U.S. Navy carrier battle groups access to the Taiwan Strait is the most likely purpose of this latest iteration of the very adaptable Dong Feng 21 (which has also been used as a Submarine-Launched Ballistic nuclear Missile, a Medium Range Ballistic Missile - with a conventional, chemical, or nuclear warhead, and as part of an anti-satellite weapon system).
An operational Chinese ASBM is a game changer unless and until the U.S. Navy comes up with some effective countermeasures or a new way to project power in the region. Between the polite strangle hold (err...leverage) the Chinese have on the U.S. economy and an ability to to influence, if not reshape, American naval doctrine - without resorting to nuclear weapons - perhaps Taiwan had better give some thought to its eventual reintegration into the People's Republic.
UPDATED TO ADD: Here's an interesting article by Stuart Koehl at the Weekly Standard which evaluates the threat of the DF-21 differently. Notably, Koehl is confident that the U.S. Navy will retain it's power projection advantage despite China's growing technological prowess and a home field advantage. Of course, such a view would seem to be in alignment with the Standard's conservative outlook.