When Vasili Mitrokhin, a disillusioned KGB archivist, defected to the West, he brought the most incredible KGB archive. So, what did we do with it? Publish it openly! A fantastic PR coup, and a very entertaining read.
Christopher Andrew keeps his biases nice and subtle, so it's quite easy to read at face value. Even taking the anti-KGB angle into account, it's quite clear that the KGB and indeed entire Soviet political hierarchy were completely nutso.
Both before and shortly after WWII, they had absolutely fantastic penetration of the West with agents, mostly because while we'd been fighting Germany, they'd been paranoid about all foreign countries. From there, it was pretty much a continual decline, thanks to a combination of executing vast numbers of people during the Great Terror, and an inability to recruit ideological agents once it was more clear that Russia wasn't exactly a worker's paradise.
However, working from such a high initial level, and with such resources at their disposal (the KGB being so much more central to their government than the equivalent bodies in the West), they were still very, very good at intelligence collection, both in politics and in science and technology. Andrew's thesis, well argued, is that the same paranoia that made them so good at collecting political intelligence also made them incredibly poor at analysing it - anything which went against pre-conceived notions (e.g. perhaps the West wasn't planning a surprise nuclear first strike) was not passed on. On the science and technology front, they stole an awful lot of knowledge, but couldn't easily bring it to production in their command economy. On the other hand, so much Soviet military technology was essentially stolen that in some senses the US was more-or-less having an arms race with itself!
Reading about their readiness to kill or kidnap people on foreign soil, their oppression of dissidents (simultaneously absurd and brutal), and generally their completely nutty priorities, it's difficult to disgree with Andrew's view that understanding the history of the KGB is key to understanding the USSR, and that it gives clear insight into a horrendously dysfunctional government.