I’d really like an informed, impartial answer to this question. To date, here’s is the best we can do:

“We have not identified a single instance involving a threat to the United States in which the program made a concrete difference in the outcome of a counterterrorism investigation. Moreover, we are aware of no instance in which the program directly contributed to the discovery of a previously unknown terrorist plot or the disruption of a terrorist attack. And we believe that in only one instance over the past seven years has the program arguably contributed to the identification of an unknown terrorism suspect. Even in that case, the suspect was not involved in planning a terrorist attack and there is reason to believe that the FBI may have discovered him without the contribution of the NSA’s program”.

This comes from the January 2014 report of the US Privacy and Civil Liberties Oversight Board, an independent bipartisan agency within the US government, which carried out an investigation into two NSA surveillance programmes in the wake of the Snowden revelations.

From an extraordinary account of a walk around central London:

Suspicion is a global variable. Once triggered it bubbles upward through the entire system. Walking down Park Lane, I was accosted by a man in a suit who demanded to know what I was doing. He took out his mobile phone, pointed it at my face, told me he was going to “circulate my description”.
Shortly afterwards, a colleague of his physically restrained me and called the police. Both men worked at the Grosvenor House Hotel, whose cameras were among those which had been trained on me as I walked, and so are included in my documentation.

When they arrived, the police officers explained that carrying a camera in the vicinity of Central London was grounds for suspicion. I might be a terrorist who posed a threat to the good citizens of London – my own city. Equally I might be casing the joint for some future crime, studying its defences in order to circumvent them.

Carrying a camera thus justified the suspicion of the security guards who stopped me and performed a citizen’s arrest, detaining me until the arrival of the police. This suspicion in turn justified the actions of the police, who threatened me with arrest if I did not identify myself and explain my actions. For carrying a camera, I was told, I could be taken to the station and charged with “Going Equipped”, a provision of the 1968 Theft Act which determines the imprisonment for up to three years of anyone carrying equipment which may be used to commit a burglary.

This morning’s Observer column:

A headline caught my eye last Tuesday morning. “Privacy not an absolute right, says GCHQ chief”, it read. Given that GCHQ bosses are normally sensibly taciturn types, it looked puzzling. But it turns out that Sir Iain Lobban has retired from GCHQ to spend more time with his pension, to be followed no doubt, after a discreet interval, with some lucrative non-exec directorships. His successor is a Foreign Office smoothie, name of Robert Hannigan, who obviously decided that the best form of defence against the Snowden revelations is attack, which he mounted via an op-ed piece in the Financial Times, in the course of which he wrote some very puzzling things…

LATER The Economist has a curiously wishy-washy piece about this. It recalls the row, many years ago, about the Clipper chip and points out that it isn’t just the GCHQ boss who is critical of the companies. Michael Roberts, the new NSA director, last week said much the same thing to an audience in Silicon Valley. As to what will happen, though, the Economist is uncharacteristially uncertain:

Although the shrill rhetoric on both sides suggests the opposite, it seems mostly a negotiating tactic. Mr Rogers’s speech in Silicon Valley was essentially an offer to talk. “I’m not one who jumps up and down and says either side is fundamentally wrong,” he said. “We have no choice but to come to an agreement,” says the boss of an American technology giant. A deal would be welcome, but only if the rules are transparent, enforceable—and apply not just to American agencies, but to the other members of the “Five Eyes”, the intelligence alliance which also includes Australia, Britain, Canada and New Zealand.

Will it happen? More likely, there will be muddling through—just like after the Clipper chip. Technology companies will negotiate some arrangement to satisfy information requests by governments. And intelligence services will try to exploit vulnerabilities in encryption technologies or create backdoors surreptitiously. Until, perhaps, another Snowden comes along.

Read on