String Theory is weird. First off, its history makes it sound an awful lot like a strange science cult. Before anyone had really figured anything about it, a ton of physicists started calling it the Theory of Everything, and, with completely straight faces, declared that by the late 1990s, all open physics problems would be solved by the theory and we would know all there is to know about the operation of the universe. This is the big one, folks! These were very smart people, and they were very much not kidding, but it's hard not to see such messianic grandiosity as anything other than a precursor to a serious LSD freakout. Which is too bad, because it made a lot of other really smart people unnecessarily suspicious of the theory, treating it like a double-Ph.D. version of Scientology.
Secondly: there is no proof. (Unless something has happened since the book's last printing.) Nada. String theory has made not one single experimentally testable prediction, which normally would put you in the mind of something like, you know, creationism. There is some circumstantial evidence, but that's about it in the proof department, mostly because its predictions are far beyond our technical ability to probe. CERN is cool, but some of String Theory's predictions would require a CERN larger than the known universe to adequately test. But string theory makes so much mathematical sense! It's basically an infrastructural theory that, when applied to all our other known theories, makes them make more sense and operate in greater harmony with one another. It makes all our other theories incredibly cool. It lines up with so many other things, so perfectly, that although it's technically possible that it's merely a remarkable mathematical coincidence that has no actual relation to our physical universe, that seems far-fetched. More far-fetched than a universe entirely composed of very tiny oscillating loops of string? Maybe not!
The author warns there are some parts that will be slow going by virtue of their abstraction and the sheer weirdness of the universe when you start messing with things on a scale far beyond that which with we are familiar--the very small, the very large, and the very fast. And oh yeah, he ain't kidding. In fact I found my self lumbering through the entire book, partially for this reason, but also because its illuminative power is at times so sublime I felt like I was truly internalizing and understanding concepts of which I had previously been aware, but never really understood. I knew these as things you might reel off over a pint of stout, talking to someone you met fifteen minutes ago: "Well of course you know Einstein's special relativity is fundamentally incompatible with quantum mechanics." But if anyone asks you why this is a big deal, or why they're incompatible, you got nothing. (a1: Because they are the two biggest theoretical tools for understanding our universe, and as currently formulated they cannot both be right, a2: because very tiny quantum fluctuations in gravity break the equations of special relativity and render them useless.)
For the first two, three hundred pages while Greene gives you the theoretical background necessary to start wrestling with Superstring Theory, the book is a like having a ray of sunlight shooting directly into your brain in a way that other, similar books (e.g. A Brief History of Time) only seem to have cast further shadows. It's a slog, but rewarding and not too painful. Once he starts outlining string theory? Ouch. Deep hurting. Even moreso than the weirdnesses of quantum theory, string theory is just impossible to visualize. For example, the idea that the universe consists of not just the three dimensions of spatial extent and one of time that we are all familiar with, but an additional seven spatial dimensions, very tightly and miscroscopically curled up. And these dimensions might undergo very complicated topological transformations, tearing and perforating, which affect the underlying laws of the universe? How does a very tiny higher-dimensional space rip? It's just space. Oy. But yeah, amazing stuff.