I agree that the secrecy behind NSA surveillance sets a bad precedent for future policies, and so the public should be kept informed. I also think that it is detrimental to foreign relationships when we do it outside of U.S. jurisdiction. However, it could easily stop a terrorist group that communicates or researches on the internet, and so should be kept intact to prevent another Boston marathon.
No, the NSA is not preventing major terrorist attacks because the last major terrorist attack came from a nation outside of the US that the NSA is not actively monitoring. Had the NSA been proven to be monitoring the entire world, perhaps a case could have been made, but since they are not, no, they are not preventing major terrorist attacks.
I believe that in general, because the NSA is not transparent with the public, people have lost faith in it. Because there is no transparency, there is no proof that the NSA have stopped any major attacks. So now there's one question that's been bothering people, Why was the NSA spying and why was it increasing surveillance if there wasn't any proof of any threat. More importantly, what have they been doing with that information if not stopping attacks.
Since 9/11 there have been 50+ accounts of attacks on US soil, not to mention what is going on in the Middle East. The NSA has stopped none of them. This makes them less credible when they say they have protected us against attacks. Where as in reality and evidence is that there have been no preventions.