Of the three, the only one I have not stepped foot in is the DAR Library. Unfortunately, I'm still looking for a relative who was involved in the Revolution. Maybe someday
While I haven't done research in the Library of Congress I have taken a tour of it. If you have never visited, let me tell you that it is beautiful inside! The architects who designed it wanted to demonstrate that Americans could build something in the classical style that compared to the great buildings of Europe. If you want a break from your research I recommend taking a guided tour of the LOC.
(Sonja Hunter, copyright 2001)
I have, however, done genealogical research in the National Archives. In fact, this is where I got my feet wet. This was back in the days when the only indexes to census records (1880-1920, for heads of household only) were soundex cards on microfilm. For everything else you had to guess where your ancestors lived and scroll through page by page and hope you got lucky.
It is probably needless to say that NARA has a large collection of records on microfilm in addition to the many paper records they house. The microfilm catalog is available online.
As I had only recently begun my genealogical journey when I was there I never delved into any but the most basic records. The notable exception was the Civil War pension application files (for Union soldiers) that I was fortunate enough to review. These records can provide a goldmine of information. Of the thirteen or so files I have looked at, probably 2/3 of them were thick files (over 100 pages). Yes, of course, there are many doctors' statements describing the ailments of the soldiers, but these records possess so much more. You will also find depositions from individuals who knew the soldier before his service to state that he was hale and hearty before the war. There are also depositions from the soldier describing how his ailments are linked to his service. Widows' pensions have even more information including death certificates, marriage certificates and birth records for minor children.
To view these records you will first need to obtain a NARA research ID card (and watch a short presentation on proper handling of records). You can register for your card in the microfilm reading room, a process that takes about 20 minutes. Card in hand, you then need to fill out a form containing information about the soldier whose file you wish to view (e.g. name, unit, application number). This information is found on pension application index cards that are now available at FamilySearch, Ancestry.com and Fold3. You can request several files in one pull and I would recommend this in case some files cannot be found. Retrieving your soldier's file usually requires several hours.
To peruse the files you will go to the reading room. To ensure that documents from NARA stay at NARA you are not allowed to bring in anything that could conceal original documents. Computers, loose notes and wallets are allowed. Blank paper and pencils are provided. Lockers to stow your other items (coats, purses, backpacks, etc.) are located down the hall and cost a quarter. Be sure to see the specific regulations on what you can bring into the reading room.
Depending on how thick a file is and the type of information contained therein it may take a couple of hours to go through the file and make your copies. If you can refrain from reveling in touching documents your ancestor did and sort without getting sucked into the depositions you may be able to get through the records faster than I did. If you are running out of time you can return the file to the attendant and resume your perusal the following day.
Don't make the mistake that I did and be stingy about what you copy. When in doubt, make a copy. After all, you already paid for this trip. While you can obtain these records by mail it costs $75 for the complete file (up to 100 pages) and a per page rate beyond that. To make copies, leave your rolls of coins at home. Your researcher card doubles as a copy card. Machines to add money to your card are located within the research rooms. However, you may want to bring several denominations (or a credit card) because you cannot get a refund if you don't use all the money on your card. Copy prices are 25 cents for paper to paper and 50 cents for microfilm to paper.
Occasionally, the NARA staff is unable to locate a particular pension file. In this case, you may want to request the Compiled Military Service Record for that soldier. While these records usually do not contain very much information, they sometimes possess more than the standard fare. Don't get me wrong, these records are valuable, but knowing that an ancestor was present for duty in July of 1863 only tells so much, though it may help to determine if your ancestor was in a particular battle.
If you go on this trip, I hope you uncover all sorts of great information and if you're lucky, break down some brick walls as well.