Faithful
These are generally quite faithful to the original stories by Sir Arthur Conan Doyle. There are a few exceptions (The Eligible Bachelor, The Master Blackmailer, and one or two others, for example) but they are usually quite true to the original stories.
A few stories that were left out, include A Study in Scarlett, and the Valley of Fear, but even with these omissions, Granada TV has created a marvelous series.