Iran, China, and Russia have all back-engineered it:
https://www.youtube.com/watch?v=o9kPMROh48M
https://www.youtube.com/watch?v=zoh2wnaP1Vw
https://www.youtube.com/watch?v=ldF0MF4Pfls
There's no telling how many secrets have fallen into enemy hands:
http://tinyurl.com/jyg7jgl
"Either the USAF has a secret UCAV capability, but only in relatively tiny
numbers, which handicaps many of the concept’s innate advantages, or the
alternative is even worse; the USAF has not pursued the technology to any
significant degree at all. Even if the better of these two possible
realities is true, the veil of secrecy surrounding such a classified UCAV
program has likely resulted in highly skewed procurement and strategic
decisions that we may not be able to recover from for many decades."