The title says it all, the 1939 Register is open.

Edit:-
Just to follow up:
For those who have a discount code, you have to enter the code before you go through the purchase process. There's a box to the bottom right which - at least on my laptop - was below the 'fold' of the page, so I missed it at first.
Enter your code there, and the middle of the three options for packages (5 households) should change in price. There is also a 15 household package available, but my code only changed the 5 household package.
Another thing to watch out for -
I received my code for the Irish version of the site (I often log in to different versions so that defaults make life easier for me). I tried using the code on the .uk site and it said the code had expired. Once I'd gone to .ie, it worked without a problem.
The image is of the whole page, though names that haven't been cleared are redacted. This bodes well for people researching streets of villages with some careful name selection.
Aside from the expected information, the image shows one column of the second page of the spread. There are notes here that appear to all be regarding later war service. On my page I have, an ARP member, a WAAF, a member of the Royal Observer corps, and a member of the Merchant Navy Reserve.
Where someone has married afterwards, maiden names are crossed out and the new name written above.
I'm yet to explore the other features, but will update so it's laid out for you all when you wake up.