Judging from Goodreads reviews, The Valley of Fear takes a lot of flak. It's criticized for having the same style of flashback as A Study in Scarlet and for ignoring Sherlock Holmes for half the story.
However, I thought the American flashback scenes were excellent. They reminded me a lot of Dashiell Hammett; I wonder if he was influenced by Arthur Conan Doyle? At any rate, Doyle's background in spiritualism is clear: his characters dabble in secret societies and cults, and the story benefits from it. This is one of the few stories that actually scared me while I read it.
The Valley of Fear is a worthy addition to the Sherlock Holmes canon and to the bookshelves of detective fiction fans.