This was solid and, considering its epoch, scary. I remember being a teen when this came up, and with U.S./Soviet relations at a nadir, nuclear war films were just starting to appear on the cinephile's radar once again, after taking a breather from their 60's heyday for the sake of the glut of natural disaster films that hit theatres in the interim.
This was one of the first and best, simply because director Nicholas Meyer was so good at sci-fi. I fervently wish he had made a lot more movies.