Declarations of Intent to become a citizen are a good source of genealogical information, if your ancestor went through the process after Sept 26, 1906.
Before that date, the Declaration of Intent looked something like this, and contained the following information: Name, age, country of origin, date of declaration, and signature.
The St. Louis Genealogical Society has a database for the pre-1906 naturalizations conducted in St. Louis, providing one with the court, volume and page number, along with several locations the naturalizations can be found.
Most of my ancestors went through the process prior to 1906. The two exceptions were the Newmarks and the Deutsches.
The Deutsches would have gone through the process in Chicago courts. I already have my grandfather's copy of his Application for Citizenship, which contains even more information, as well as his birth certificate from Varalmas, Hungary, so there isn't much information I would find on his or his father's Declaration of Intent I don't already know. The one exception would be his father's birthplace, which could have been a different town than his children's.
The Newmarks went through the process in St. Louis courts, so their declarations are easier for me to obtain, being on microfilm at my local library. I'm kind of surprised I waited this long to retrieve them since I've been to the library so often, but I've been busy with other records.
The St. Louis County Library website has the post-1906 naturalizations in St. Louis indexed by volume number. If you don't know the year to look in, you can search the entire site using the search box at the top of the page. The index provides the specific court, declaration number, volume, page, and both the library's and LDS microfilm number.
This is what the post--1906 naturalizations look like. (Click to enlarge) It is my second great-grandfather's Samuel Newmark. It contains:
Name, Age, profession, physical description (eyes, hair, complexion, height, weight), place of birth (city, country), birthdate, last foreign residence,
current address, port of departure and arrival for immigration, name of vessel, date of arrival, and signature. (Or a mark, and the signature of whoever witnessed the mark. In the case of my great-great-grandfather, one of his sons.)
This is much more useful to the genealogist than the pre-1906 forms. After 1916, two more pieces of information were added: Name of wife, and the country where the wife was born.
New information for me included his date of birth (Oct 2, 1863). His tombstone had said 1862, with no month or day. His town of birth was also new information: Wurka, Poland. A search at JewishGen led me to Warka, which is part of the Warsaw province. Most of his children put down Warsaw for their birthplace, and that is what had been passed down, though I suspected they hadn't actually been from the city, but one of the rural towns nearby.
My great grandfather's declaration of intent was also fun to look at. As I've mentioned before, Barney claimed to be from Dublin and to have been born on March 17th. This made a lot of business sense in a town filled with Irish immigrants, and he could explain his accent by mentioning he spent 14 years in England. The suggestion one commenter made awhile back that my great-grandfather may have gotten the idea for his 'born in Dublin, Ireland' fib from Deblin, Poland is getting more interesting. If you look at the map on the first link in this paragraph, you will see Warka, Poland is half way between Warsaw and Deblin.
His 'born on March 17th' fib is also looking less like a fib. Barney definitely did say April 14th for his draft registration, and that's the date that his wife gave for the death certificate. However, his family celebrated his birthday on March 17th, and on his declaration of intent in 1910 he said he was born on March 25th. 8 days after March 17th. This could be significant since in Jewish tradition, 8 days after birth is when a young boy is welcomed into the Covenant with G-d.
Perhaps there was some confusion at some point - maybe due to the need to convert from the Hebrew calendar to the Gregorian - and Barney's birthday was computed using the wrong Hebrew date. And later this was discovered. This explanation might not be extremely likely, but still a possible defense of his claim. Still, the earliest document now found puts his date of birth in March, not April.
Another interesting discovery is that Samuel's youngest son, Israel David, declared his intent in 1922, at age 19. But I don't know why he needed to. Minor children automatically naturalize with their parents, and Israel would have been 7 years old in 1910 when Samuel began the process. I don't know the date Samuel actually became a citizen, but the process wouldn't have taken long enough.
For access to Naturalization records from other courts, if you can't find the records locally, FOIA requests can be filed with U.S. Citizenship and Immigration Services.