Well, the big problem is that the default state of a secret spy agency is to deny everything and to classify everything. So their denial means very little. I would expect them to deny what they could. GG may, in fact be right in part but wrong on some details; he may be right in principle but wrong in fact; or he may be wrong, period. The preponderance of evidence of NSA behavior (going back to the 90s), the previous whistleblowers, and the admissions to date indicates that in principle, GG is correct. He may be wrong in detail or in facts, but his story is generally consistent, if more than expected.

Of course, it would be nice if Congress forcibly opened the archives and let the truth out into open air - then we could see clearly and make educated decisions! Until then, we have to rely on the somewhat foggy environment of leaks, whistleblowing, and journalists.