A useful resource, but it would be more useful if there weren't quite so many transcription errors, at least on the Findmypast web site. Some of them are really crass: not just misspelled names but incorrect gender or marital status, and also addresses (admittedly Salutary Mount is an unusual name for a street, but "Sanitary Mount" is somewhat less probable).
I urge everyone with a Findmypast subscription to look up ancestors and relatives who were alive in September 1939 (if you haven't already done so), and to report any errors in the transcript. To do that, click on 'More actions', 'Report an Error', and don't forget to 'Submit corrections' at the foot of the page. Assuming the corrections are then made, that should help others to find the relevant people.
Note that there are some errors in the original, including a few incorrectly declared dates of birth;
Findmypast won't alter those. And I suspect that men who were already serving in the armed forces had military ID cards and were not generally included in the 1939 Register -- though I did find my father who was in the army but visiting his parents (perhaps on pre-posting leave) and my mother who had just joined the WAAF and was doing clerical work at an RAF aerodrome.
It's a matter of luck whether anyone born less than a century ago is included.
I found one cousin born in the 1930s and still alive, though the record for his deceased sister is "officially closed".