These Registration cards have truly been invaluable to my research. The main reason - the records are first hand sources. Each record is coming straight from the man that filled the card out, not a census taker that can't get the spelling of a last name right. Granted, due to their age, sometimes the card might be victim to bad memory.
Another thing that is really neat about these records is they serve as a snapshot of the person. There is a section for the draft board to list the description including their height, weight, hair color, eye color, and any abnormalities.
Below are three records of my ancestors:
John Bernard Marchel - Maternal Great-great Grandfather
Edward Morrison Kline, Sr. - Maternal Great-great Grandfather
George Leo Rak - Paternal Great-great Grandfather