It's a fantastic exhibition, really clean and crisp, with a huge selection of propaganda posters dating from 1914 onwards, from the UK, Europe, America and Russia. There are a few classics that you'll recognise (Britons wants YOU; I want you for the US Army; Daddy, What did YOU do in the Great War?), and a whole host of others that you won't have seen. They've also published a fantastic book of the show by Dr James Aulich (who curated the show).
The range of posters stretches up to the present day, with work by David Gentleman (who also created the show's identity, working with advertising agency Ogilvy) and Peter Kennard; but it's really the earlier stuff that captures you with its power, simplicity and beauty.
Read more about the show in this article from Time Out magazine. The exhibition runs through to March next year, and it's free entry, so you've got no excuse for not getting along there.