I will be happy to engage Drexler at length when I get the chance to do so. I have not, in the last 3 days, managed to buy the book and go through the physics in detail. I hope that failure is not enough to condemn me as not acting in good faith.

Absolutely not, and I think this occasioned a useful discussion. But if you have a physics or chemistry background, I for one would greatly appreciate it if you did so (and the Smalley critique, and perhaps Locklin below) and posted your take. Also you don't need to buy the book, you should be able to get a copy at any large university library.

Richard Smalley is the canonical white haired Nobel Laureate who disagrees strongly with the idea of MNT as Drexler outlines it.

I am no expert in the relevant science, but I take the Smalley argument from authority with a grain of salt, for two reasons.

First, according to wikipedia Smalley was a creationist, and apparently he endorsed an Intelligent Design book, saying the following:

Evolution has just been dealt its death blow. After reading Origins of Life with my background in chemistry and physics, it is clear that biological evolution could not have occurred.

If he underestimated the ability of evolution to create complex molecular machines, perhaps he did the same about human engineering.

Also, the National Academy of Sciences, in its 2006 report on nanotechnology, discussed Drexler's ideas and did not take Smalley's critique to be decisive (not a ringing endorsement either, of course, suggesting further experimental research). Here is a page with the relevant sections.

This critique by Scott Locklin seems mainly to be arguing that Drexler was engaged in premature speculation that was not a useful contribution to science or engineering, and has not borne useful fruit. But he also attacks nuclear fusion, cancer research, and quantum computing (as technology funding target) for premature white elephant status, which seem like good company to be in for speculative future technology.

He says that there may be technologies with similar capabilities to those Drexler envisions eventually, but that Drexler has not contributed to realizing them, and suggests that Drexler made serious physics errors (but isn't very clear about what they are).

I would be interested in knowing about the technological limits, separately from whether they will be reached anytime soon, and whether Drexler's contributions were any good for science or engineering..