It's worth quoting from the Monthly Film Bulletin's review at some length:
This is certainly one of the most interesting documentaries of recent months. One of the most stimulating things about it is a complete absence of propaganda... It is not concerned to present an argument of any kind. It stands simply as a record of those first days which we remember so vividly [the review was published on 30 November 1939, so those days were hardly ancient history!] and so achieves the rare distinction of being a documentary in the real sense of the word. Inevitably it shows scenes with which we are all familiar: yet they are presented so strikingly and realistically that the familiar becomes miraculously exciting... Yet despite all this it is a major fault in the film that it is defective in construction. It lacks unity. One is left with the feeling that either it was not conceived as a single whole from the outset, or else the unity was lost in constant alteration during production...
Indeed, the circumstances of the shooting, like so many early wartime logistics, were a little haphazard... The shots were directed by no fewer than three of the GPO Unit's most characterful filmmakers - Harry Watt, Humphrey Jennings and Pat Jackson. In their later recall, the film was a semi- maverick product of the unit's boredom and frustration while waiting for civil servants to decide what to do with them, but in fact film historians have shown that the project was, in principle, officially sanctioned. Stylistically, it comes across as a patchwork of several of the styles they had been pioneering, and those to come: plainly explanatory, awkwardly dramatised, socially inclusive, beautifully impressionistic. In particular, Jennings fans tend to see it as presaging his later, more formally perfectionist Listen to Britain style, but it's better to enjoy the film in its own right, as the product of its own peculiar 'phoney war' moment.
A sense of strong emotion rising behind stiff-rigid upper lips is palpable. The script's 'childhood' metaphors are almost more moving for having been too sketchily worked out, the 'children' of the First World War now the young men mobilising (today, of course, they are great-grandfathers). Another great strength is an unquestioning faith in the power of the static photographic composition - the faces of parting lovers, and eerily empty streets alike, were rarely more eloquent.
*This film is included in the BFI DVD compilation 'If War Should Come: The GPO Film Unit Collection Volume 3'.