Artificial Intelligence vs The Human Brain

Are the warnings we have heard, from no lesser figures than Bill Gates and Stephen Hawking, about artificial intelligence being a threat to the future of humanity, to be taken seriously? Will our children or grandchildren live in a world where walking, talking humanoid robots are difficult to tell apart from biological humans? There are those in the field, such as Ray Kurzweil, American writer and computer scientist, who believe that in just a few decades machine intelligence will be able to improve itself, independent from human help, to generate exponential improvements that are described as "the singularity" - a term borrowed from mathematics where it is used to describe a function that veers off to infinity.