Looks bad:
Confidential report lists U.S. weapons system designs compromised by Chinese cyberspies
But if you want to be cynical, it's worth noting that claiming the Chinese have stolen the designs of our most advanced weapons plays directly to the interests of the military-industrial complex: "OMG, if they know about today's best, then we have to spend billions more developing weapons that are even better!"
Of course, there's a precedent for this: the same military-industrial complex issued panic alarms about how the Russians were surpassing us militarily during the Cold War, when (we later learned) they weren't even close. That's how the defense budget piled bloat on top of bloat.