Great point about memoirs!
None of my family members ever went to the lengths of writing memoirs, but I do have two single sheets of paper that were written by my ancestors in the early-1990s that have proven to be valuable in my current research.
The first is my grandfather's account of how my great-grandparents met that my cousin wrote down in the early 1990s:
While some of the details are clearly exaggerated/fictionalized, the story of how my great-grandparents met matches up perfectly with the marriage records I have found for them. Amazingly, the list of the 13 children (and many of the details associated with the children) matches up perfectly with everything I have found in birth and census records. In fact, the list in this paper laid out the exact set of 13 names that I needed to find. I have found a nearly complete set of birth, death (and marriage records for those who lived) for all 13 of the children listed.
The second paper is my Great Aunt's own handwritten notes on her father:
Despite a long and exhaustive search, I have been unable to find the name of the German town where my great-grandfather was born. My Aunt Bertha's half page of notes on her Father have been invaluable to me as I try to search for him. Like any second-hand account, I have had to challenge and attempt to verify every detail with official records. And, while I haven't tracked down his actual birth record yet, I remain optimistic as this paper has given me many names to try in searches.
Similar to Bob and Eowyn, I wish that I had "memoirs" of any form for my ancestors. In the two examples above, just having written stories that I can attempt to study and verify really makes it engaging and challenging in my long pursuit to track down the birthplaace of my great-grandfather.