Nowhere near as bad as you might expect. In fact, quite entertaining. Bearing in mind it was made in 1961, the special effects (e.g. of a dried up Thames and a London shrouded in mist) are quite good. However, the premise remains a bit silly. Janet Munro had a nice arse.
Probably Val Guest's finest film and also probably one of the finest sci-fi films the UK has ever produced. Thoughtful rather than paranoid, and tense rather than panicky, it offers a delightfully British spin on the 'nuclear bombs have knackered the world' sub-genre. If possible, track down the BFI restoration as it's a beautiful piece of work on a splendid film.