History Graduate Students' Union
The most important thing to remember when dealing with the US federal archives is that they're not actually in D.C. Yes there are archives there but they're mostly a museum at this point with only a very limited selection of documents. Most material (including Military and Foreign Affairs) is located in College park MD, about 30-40 minutes north of D.C. The ARC finding aid system should tell you where the documents you want are.
Strengths
Archivists: The US archives lack the online finding aids found in Canada, they compensate with a large and knowledgeable staff of archivists that are constantly helping clients find material. Asking one of these people what you should be doing and where you should be looking is a vital first step.
Fast & Efficient: Documents are brought out every 3 hours and a hard working researcher can go through an enormous amount of material in a day.
Weaknesses
Paranoia: Be prepared for scans, searches and multiple checkpoints to get in or out.
Finding Aid System: Most NARA searching is done with paper finding aids. There is an online system called ARC - however this system does not provide you with reference numbers and cannot be relied upon to be an exhaustive search of your topic. Before visiting, a researcher should go through the ARC system and find as much as possible using keyword searches - these searches can then be used as a point of departure when using the paper finding aids.