I know pretty much nothing about superstrings, so when I saw this popsci book in Oxfam, I bought it. Having done so, I mentioned it to a physicist friend, who said , 'The Cambridge book? Not very good, is it?'. I think this is not terribly fair. It's not really a guide to the theory itself, but rather interviews with top theorists about string theory. It's the book form of a radio series. There is a big chunk of introductory text which explains the ideas behind the theory, but mostly as a background to the interviews, rather than as a guide in itself.
Given it's a set of interviews covering miscellaneous topics in the area, the book's 20 years old (interesting reading the discussions of what the LEP might bring!), and the topic was (is?) very much in the air in terms of understanding, it's perhaps best viewed as a piece on the history of science! From this point of view, it's very interesting, effectively being about how scientists deal with uncertainty in their models - trying to find ways of testing new theories, and cope with theories that they can't actually deal with mathematically at the moment.
It's from this side that it's particularly interesting. String theory is given as a way of fitting gravity into a quantum theory, but the calculations being done were approximations that really don't actually exercise that. It was a big hope to eliminate anomalies and produce finite results, but no proofs had been produced. It was hoped to explain the masses of particles, but at the time could not helpfully describe interactions at energies we're used to.
Given all this, it sounds like a lovely theory, but one that finds it hard to claim a grip on reality. I can very much understand Feynman's careful skepticism. On the other hand, the book's very good at selling the beauty of general relativity!