To be honest, I feel kind of guilty to have enjoyed "The Sign of Four". During the first half of it, the novel was not nearly as good as "A Study in Scarlet", but then everything twisted and turned and tampered and I was basically so hooked I couldn't take this story down. 
 One advantage of Arthur Conan Doyle's writing in this novel has certainly been his decision not to include a random plot switch like he did in "A Study in Scarlet". The mystery's solution was actually told through John Watson's perspective rather than within a long-winded narrative about the murderer. Thus, the reader is able to follow Watson and Sherlock through the entirety of this novel, and yet ... the beginning was boring as hell. I know, I know. We had a murder. We had a treasure. We had Mary. We had John. We had Sherlock. But ... whew. 
 To explain my feelings about this book more precisely: The first half included a lot of different interesting aspects, e.g. Sherlock's explicit use of cocaine as well as his ways to explain why he takes it, and the introduction of Mary Morstan. But I couldn't get rid of the feeling to be alienated from Watson, as though it wasn't really possible to get inside his head and take a look not only at the murder case, but also at the interesting characters surrounding this case, namely Sherlock, John himself and Mary. I think I like Mary's TV version with the brilliant Amanda Abbington more, because Arthur Conan Doyle kept her character on a rather one-dimensional level. In addition, the fast-building romance between John and Mary was weird to witness as a reader, because John's behaviour seemed quite irrational and rash, even for him. 
 On the other side, Arthur Conan Doyle was able to write some breathtaking scenes, including a pursuit and an interesting perspective on the Indian Rebellion - but the plot was mostly foreseeable from the beginning of the second half on, so the story was never able to really surprise me. 
 In conclusion, it was a good novel and offered an enjoyable read, but no excellent one.