I had to research this topic for debate and I learned quite a bit
Basically the NSA has maybe attributed some information to the plotting of about 3-5 terrorist attacks, only one of them being thwarted mainly because of NSA intelligence, the others basically were eventually uncovered by either foreign agencies or the FBI. But then of course the NSA sees it necessary to have programs available like Xkeyscore which allows unrestricted access to private information and easy background checks (Edward snowmen faked to be an administrator easily)
All in all the program costs billions of dollars and violates the privacy of citizens not only in the US but other countries as well, all to maybe save 200 people...should we really give up all of our privacy for a measly amount of security? I don't think so...
That too in the fact that it resorts to using BS evidence to support itself (DDOS attacks classified as cyberterrorist attacks? Seriously?)