If you ever have the chance to go to the National Archive Center I highly recommend it. It is wonderful. There are a few, I happened to go to the big guy in Washington D.C.! I did prepare in advance. I was searching Civil War pensions. I had about 8 different records I wanted to see.
I first looked up any male in my family tree that might have been at an age I thought he would have enlisted in the Civil War. (1861-1865) I then looked up every name I had at Ancestry.com.(you need a membership) I wrote down all the men I found that had applied for a pension. I found a great great uncle this way! His mother applied for his pension. You take the pension application numbers to the National Archive Center.
I had someone that worked there pull all the files before I arrived. If you don't get them pulled in advanced be prepared for a wait time to get your files. The files are wonderful. Most of them had great information in them. Some were very brief. I found in the files things like:
parents names and death dates
children names and birth dates
wounds from battles
where they were living
Just a note... I did pay for a file copy and had it shipped to me. I bought the cheaper file and didn't find the information I was looking for. I did call them and asked if there was other information that I might find useful in my family search. They were very happy to help me. They re pulled his file and looked again. They didn't notice anything. When I decided to take a trip to D.C I decided to have his file pulled again. He had a large file and in it I found the things I really wanted to find! So if you can't go yourself I recommend just buying the copy of the whole file. I am sure they didn't really know what would be of worth to me. They sent me all the main pages on him and his battles. I was looking for information about his family.
I know it is a gamble to pay for the whole file. Some really are small and some are well worth the money. If you live close enough go in person. It is free to look all you want. You only have to pay for the copies you want. Go prepared.
I wish I lived close enough that I could go often. They have all kind of records stored there! It is a searchers dream come true!