My understanding of the nature of these records is that they were basically the complete list of all phone calls and emails made and received by all United States citizens in the last year or so. The U.S. government hoped to be able to identify patterns that would be of use in spotting terrorist attacks in the planning stage. I think this would work only after they have identified suspects that they wanted to keep an eye on. The government has been strangely silent in identifying even a single terrorist attack that was thwarted by the use of these data (that information is too hush-hush to share with the American public, no doubt.)
That is the part that scares me a little, together with the colossal scope and scale of the operation. The position of the government is that the people can never question what the government does, because it would be a disaster if government secrets came out. That much is true – if all the secret shenanigans of the government were made public that would indeed be a disaster for public office holders.
In a democracy the people have a right to know what their elected leaders are up to.