The best thing I’ve read about the report is by Julian Sanchez. An excerpt:
The heart of the Horowitz report deals with the Carter Page FISA application, and documents a progression that ought to sound familiar to anyone who’s studied the history of the intelligence community: An investigation begins with a kernel of reasonable suspicion, and facts are marshaled to support a theory. As it gathers momentum, those initial suspicions congeal into assumptions. New information that fits the original theory is added to pile of evidence—while a growing body of contradictory of information is overlooked. It’s possible to read the Horowitz report and think that the initial 90-day wiretap of Page was justified, but far harder to rationalize intrusive surveillance that carried on for nearly a year, through three separate renewals, even as evidence mounted that should have undermined the basis for the warrant. . . .
[T]he picture that emerges from the Horowitz report is not so much sinister as banal: The government asked the court for “one more go” essentially out of inertia. Case agents weren’t motivated to think terribly hard about whether the most recent piece of information they’d uncovered contradicted a claim they’d made to the court months earlier. Higher level attorneys reviewing renewal applications focused almost exclusively on vetting the new information in each filing, never going back to reexamine earlier assertions and test whether they were still defensible. Verification meant checking the files to validate fresh data, but not to take a fresh look at early assumptions. After all, someone had checked that already, right?
This picture is, in its own way, and for very different reasons, as disturbing as the image of a Deep State cabal with a vendetta against Trump: Vendettas are at least specific. Whereas the grave defects in the surveillance of Page seem more likely to be symptoms of a more apolitical, and therefore more systemic, form of bias. Their underlying causes—reliance on sources whose claims are hard to directly check, imperfect information, case agents making judgments about which facts in a vast sea of data might be legally material—aren’t peculiar to elections but endemic to intelligence.