Ancestry.com released the complete 1940 census index today and I've spent a little time experimenting with it. Like any index, it will have errors and issues, but the option is to search page by page. This is slightly impractical when research urban ancestors for whom no good address is known. I'll avoid opining about errors in indexes. Indexes are meant to be finding aids, not definitive transcriptions. And the subscription to Ancestry.com would be even more if they paid professionals to create the indexes instead of taking the approach that is apparently taken.
This is part of the entry for Marie Williams, my wife's great-grandmother. Marie lived in Chicago in 1940 with her third husband, Wayne Williams.
Without the index, it would have taken me forever to find them. Williams is not the most unusual name.
Google Maps indicates that there is still a home standing there today.
I've got a few more 1940 people to find, but the index is helping with those who were "missing."
Remember that through 2013, you can search the 1940 for free atAncestry.com (no credit card is necessary).