Also, don't forget that humans will be improving just as rapidly as the machines.
My own studies (Cognitive Science and Cybernetics at UCLA) tend to support the conclusion that machine intelligence will never be a threat to humanity. Humanity will have become something else by the time that machines could become an existential threat to current humans.
So the real threat to humanity are the machines that humanity will become. (Is in the process of becoming.)
From the final chapter of his new book Cowards, titled "Adapt or Die: The Coming Intelligence Explosion."
The citations for the chapter include: