[vegashacker]

Probably impossible to verify one way or the other. May be more true to say "...have led to little evidence of stopping...". See, e.g., Schneier's article on this (https://www.schneier.com/blog/archives/2014/01/how_the_nsa_t...), which references a summary article on what may be true about the effectiveness of the surveillance in stopping plots/attacks (http://www.huffingtonpost.com/2013/10/23/nsa-attacks-thwarte...).

[mpyne]

NSA overall has certainly been successful in stopping attacks.

The major question was the effectiveness of 215 phone metadata surveillance in particular, which is the thing that was hotly debated. But even leaving out 215 metadata there were still about least a dozen plots with actual planning/action taken for them and probably 35-ish more that were detected and either stopped before they reached that point, or verified to have trailed off on their own.