When I read "Engines of Creation," I ended up with the same feeling you
have. I therefore bought "Nanosystems" and read it (reasonably)
carefully.
I feel that I have enough technical background to evaluate Drexler's
arguments in "Nanosystems", except for the thermodynamics. I found the
arguments convincing. I consider the proposition
A) "a MNT factory can build another MNT factory"
to be proven in theory. I consider the proposition
B) "given a MNT assembler, we can build a MNT factory"
to be proven in theory. I consider the proposition
C) "We know how to build the first MNT assembler without using MNT"
to be unproven. However, I appears to me to be likely. It's therefore an
assumption in my line of reasoning.
My discussions with Robin lead to explicit statement of the following
additional proposition:
D) "we will develop capable MNT design software."
This is not proven, so you may treat it as a premise. I'll call it a
professional judgement.
Now, My various random posts may not constitute a formal proof, but
I feel that C and D lead to A and B. I also feel that A, B, and D lead
to rapid feedback, economic meltdown, and the Singularity.