I am researching my FIL's Tietjen line and I'm fortunate that they lived in Brooklyn, NY. On Footnote.com and Ancestry.com, I have used the directories to piece together a timeline of the Tietjens. If they had lived in a smaller town without such a source this research would have been much harder.
Although the directories only give the head of household usually, sometimes it gives an occupation which is very helpful if you are trying to distinguish between two people of the same name.
So, if you cannot find people on census records, don't forget the directories!:)
Anyone else had success with Directories?