This seems to be a solid introduction to computability theory. Mind you, I am not a computer scientist, never studied computer science in school, nor do I do research in a related field. So what use are my impressions? Not much, probably. I am a physicist-turned-data scientist and am simply curious about this stuff. I didn't work through every proof, and didn't touch the end-of-section problems. If, like me, your life doesn't depend on proving that SAT3 is NP-complete, or explaining why the halting problem is undecidable while hanging upside down over an alligator tank in the cellar of some sadistic though technically-inclined serial killer, you might consider my review relevant.
Given a moderate level of effort, you will come away from this book with an essential understanding of decidability, formal models of computation, and computational complexity. The book is not the most pedagogical...it is quite formal, terse, and dry in places. The author is not in a bad mood but he is not smiling very much either. He is not excitedly welcoming you to this mysterious new land of computability theory; he's more like a jaded veteran lab technician showing you around the spectrum analyzer, gruffly telling you to clean up after yourself and to definitely *not* touch the red button. Sipser assumes you're pretty smart, and that you don't have follow-up questions. Maybe that's true of MIT students. I'm certainly not one of them.
As an example: the idea of reduction. We can 'reduce' a problem A to a problem B and use facts about the one to understand the other. If B is decidable then so is A; conversely, if A is undecidable then so is B. Now, why this asymmetric logic? Presumably it's because B is considered the harder problem (which strains the terminology 'reduce'), and if B can be decided, then the easier problem A too can be decided. But this isn't spelled out anywhere. This seems like an obvious question anyone seeing this material for the first time might ask. It could be dispelled in a line or two, but Sipser doesn't address it. There are lots of examples like this, where you're like "hey, hold on a quick second...could we just think a bit more about...no? Moving on? OK..."
In terms of structure, there are plenty of examples, but more often the structure is: introduce topic, theorem, proof of theorem, introduce topic, lemma, proof of lemma, .... lots more of this ...., then maybe an example or two. This is why I say it is not particularly pedagogical: it's not really *teaching* as much as communicating ideas. It's certainly not engaging, the way I think introductory texts should be.
Apparently Sipser is popular with people who, unlike me, take this subject seriously. For off-label use, it's pretty solid but has defects, some of which I've listed above. I suspect there might be better treatments for amateurs and idiots.