I'm a total sucker for 1970's horror and this is some of the best I've read. I saw it on a list of must-read horror works in one of my HWA writing books, and I think that its inclusion was very well-deserved. 
 It has all the usual excesses, the sexuality, the violence, and elements of the profane. It certainly feels familiar, too, because it influenced a lot of horror that came along later. Some of the sexual themes (minor spoiler: (view spoiler)[Edith's fear of lesbianism, for example (hide spoiler)]) make it feel dated, but the plot kept me guessing right up until the end. Matheson is a masterful storyteller and I love the clarity of his writing. For that, I'll bump it up to a 5 (though it's probably a 4.5 for me, if I'm being super-picky).