Have you been hitting at your brick wall for years and only scratched the mortar? DNA testing may help you tear down the wall completely.
One of the benefits yDNA testing is connecting with other researchers of which you share a common ancestor. Suppose your family lore is your GGG Grandfather immigrated to the US in the year 1805. You have found your GG Grandfather in the Richmond, VA 1850 census living next door to another family with the same surname. After much research you could find nothing more about your GG Grandfather Henry or the mysterious John that lived next door. Plus to complicate it more, by 1860 John had disappeared with his family. You test your yDNA line and discover that you share the same yDNA markers with a family that traces its line back to a John that was the son of a Richard who immigrated to Richmond, VA in 1805. John had a very detailed Family Bible that named his father and his mother, birth years and the country and town where they were born.
Another example might be that your ancestor was adventurous and left to go to California during the Gold Rush. All you could ever figure out was that he was born in Ohio in 1834. Problem is, none of the census records prior to 1850 show the family members. You know he was married at the time he left, but there are 40 men in Ohio with the same surname and he was still living with his father in 1840 because he was 16 at the time. Through DNA testing you find out that you match a 3 other descendents of a family from Hamilton Co., Ohio. After some research and collaboration, you discover that your ancestor's father mentioned him in his Will of 1847. Plus, you locate a marriage record in 1845 and know his wife's maiden name. Plus, you have three more unknown distant cousins.
Genetic Genealogy will not give you all the answers. I like to call it a map to the possibilities of a discovery. Even if you do not match anyone at the start, you may find a match in the future. From my own personal experience, I can tell you Genetic Genealogy does work and it does help focus your research. As more and more males submit to the test and join a Surname Project those brick walls are broken down.