In my experience, the U.S. National Archives website was one of the most difficult to use and poorly organized large online databases. Even if you could find the title to a record you were interested in viewing, it was very difficult to figure out how to get access to the record. Usually, the conclusion was that the record was only available for examination, in person, at the National Archives location holding the record. For that reason, when the Footnote.com website was implemented, it was a major event because the site was established to provide access to digitized copies of the National Archives records. But then Footnote.com was purchased by Ancestry.com and became Fold3.com. It is now far from clear as to how the records in the National Archives are being made available in digitized format.
This summer, the National Archives has completed changing their cataloging system from what was previously called the Archival Research Catalog (ARC) to a new system called the Online Public Access (OPA). Unfortunately, it is far from clear how this new system changes the way records are accessed or what records may or may not be available online. The National Archives site says that the OPA prototype currently contains all of the ARC data, including the archival descriptions or catalog records, authority files, and digital copies.
For example, the startup page for the National Archives has a search field. If I enter my surname into the field and do a search, I get all sorts of documents listed containing the word "tanner" and any portion of the word, such as "tan." So far, I haven't figured out how to get a meaningful response and further, how to figure out which documents are available online and which are not without clicking on each document.
I am going to spend some time working with the site and see if I can figure out any faster way of doing real research with their online records. Stay tuned.