How many times has the pervasive electronic surveillance that we live under stopped a terrorist attack? This is considering that the reasoning repeatedly given for enforcing this policy is that stopping the threat of terrorism is more important than every other aspect of society.
As others pointed out in the comments, a list of successes would not be published by all concerned agencies.
There is one example from Cologne, Germany. were information from the NSA was apprarently passed to German authorities. The German wikipedia text says so, as did news media and spokespeople at the time; the English one is shorter.