I've read about half of this collection before in various places. And I would agree that the best stories in The Long Valley are those which have been heavily anthologized. But reading them again in the context of the whole collection was surprisingly enjoyable. Here you get a wide range of Steinbeck's tone with a single theme throughout: violence.
As with all collections, some stories were weaker than others. In particular, I wasn't a fan of “The Murder,” a story which seemingly justifies the abuse of a wife. Having never seen Steinbeck as a raging misogynist, I chalk this story up to an objective portrayal of the culture at the time. Other stories in this collection may imply I'm wrong, however. We'll leave it at that.
Certainly, Steinbeck was primarily a novelist. He wasn't a masterful short story writer, but that doesn't mean he couldn't write a short story. Obviously, he could. I enjoyed this collection despite its limitations. Steinbeck fans should definitely get around to reading this one. Others may just wish to stick with the more heavily anthologized stories (e.g. “The Chrysanthemums,” “Flight”).