Oh, yeah: its a spy whodunnit. I can't talk about it without spoiling the surprise. Don't read on if you care.
The flawsThe flaws, the flaws, Carruthers! I think they are threefold: firstly, that it seems a very small world. To some extent the book begins to address, or talk around, this point towards the end: "Gerald" goes on about how small a place England has become. But what I mean is that the Circus seems to spend its whole time chasing its arse. Perhaps you can argue that for dramatic effect all other operations are elided? Secondly, it seems less original than it must once have. Perhaps it was The Original of the "there's a mole in our spy network" type story; but its a commonplace now, and that's the view I'm reading it from. Third was a plausibility problem: how does a retired spy end up subverting the whole Circus? That would be terrible security, if possible. How can Guillam so trivially steal and photograph files he's not entitled to? Oh yeah, and fourthly, I don't think the central plot is entirely believeable either: the idea that everyone was so naive as to believe an unexplained sudden flood of intelligence as genuine seems far fetched. Its also necessary that, say, Jim be so emotionally wound up after Testify that he not go talk to Smiley, which is unexplained; as it Smiley being sent away but Control during the operation. As indeed is Jim's bizarre decision to go ahead even though he knew he was compromised.
The virtuesBut, its quite a good book. Well written. Good storyline. Exciting. Once you know the answer the denouement isn't a surprise but even so I could see myself reading it again, because its complex enough that on the first reading you'll miss stuff.
Something worth calling out - though the text does point it out, several times, because it is the central cleverness and he doesn't want you to miss it - is the cuteness of the "knot": by allowing the idea that a genuine Russian source had to be fed "chickenfeed" in exchange for his high-quality stuff, it was necessary for there to be a fake-mole in the Circus, who should not be investigated; and therefore the true mole could hide behind the fake mole.