How will the advent of molecular engineering affect humankind? When can we expect such technologies to become workable? I believe the answers to these questions are somewhat different from those offered to date.
One claim for nanotechnology is that it will make everything cheap. Yet technology has already made life's necessities cheap, at least in the developed countries. The five cents I pay for a pencil is cheap enough that lowering the cost will have no noticeable effect on my life. To a lesser extent, the same is true for food and housing. Many of the expensive things in life are costly not because they are difficult to make, but because government regulations and transactional costs make them expensive. True, diamonds would be more affordable if they could be cheaply manufactured, but how important are diamonds anyway?
The real value of nanotechnology would be in allowing us to do things we cannot now do, such as repairing damaged arteries or flying a single-stage vehicle to the moon. But I believe many have underestimated the difficulty of writing software which will tell molecular assemblers how to assemble such systems. This will be much more difficult than writing software for a computer which does one thing at a time. For molecular assemblers to do useful work, billions of them must work cooperatively. This implies the need for billions of different programs.
Advocates of nanotechnology point to living systems as proof that molecular machines can be built. But living systems were never designed. They evolved. Living systems are so complicated that it might well be impossible to design one without copying the plan of a very similar system. It might be possible through guided evolution to create a new sort of molecular life, but growing such life into a space ship would not be a quick and easy undertaking.
I expect the software problem will prevent nanotechnology from making major contributions to humankind during the next fifty years. Even such challenging technological problems as artery repair and inexpensive space flight are amenable to other solutions, and given enough time, such solutions will appear.
Nanotechnology will come, eventually. But don't waste time waiting for it.