First, I really felt that two of the plot devices in these novels, one minor and one pretty major, were kind of hackneyed (more detail in the spoilers section below). Second, it was just too obvious that the novels were trying to be a bridge and explain later concepts and events. Those explanations, I think, were simply not necessary. The Foundation novels certainly stood on their own, recipients of countless accolades, without these two pre-quels. Even after the robot novels created a bit an open-ended story as to the ultimate fate of Daneel Olivaw, I don't think there was a huge need to retcon the story line. A little ambiguity and uncertainty is OK in a story of this magnitude.
SPOILERS FOLLOW:
OK, it was just obvious from nearly the beginning of Prelude that Dors was a robot. This was really just laziness on the part of Asimov, I think. She became a crutch upon which he relied to save Hari at various points. Rather than use a deus ex machina, the (famously) atheist Asimov relied simply on the machina itself.
More disturbing, though, was the recurrence of the mind-reading trope. Really? I know Asimov was at the end of his career and life, but to recycle this theme again as a means to effect the creation of the near-magic technology of psychohistory was, again, lazy.
All in all, I'm not that thrilled with the writing in these books so far. I'm interested to see how I feel about the core novels that come next.
