This is one of the most intelligent TV dramas ever. The cases themselves are highly unusual (albeit usually quite complex -- concentration necessary!), but what really makes the series great are the characters -- especially Sherlock (VERY quirky!) and Watson (Asiatic Joan -- bright and a steadying influence), along with a host of characters who make appearances along the way (some of whom disappear and reappear a season or two later -- just like in real life). Their character development is both "radical" and also "natural" for the type of persons they are. In short, fascinating "sideline" plots beyond the crime cases they have to crack.
Given that, this is not a series that you should start watching in the middle. The character's evolution is such that the greatest pleasure (and surprises) can only come from seeing the show from start to finish in mostly "chronological" fashion (although each show does start with "what happened recently" related to the theme of the current program).
My wife and I have 5 higher education degrees between us (one of us teaches Media, the other has an M.A. in Communications), so when I say that this series is a delight, I mean that in the best sense possible. One does not have to have a "high IQ" to enjoy it; just a willingness to let super-intelligent plotting/scriptwriting and terrific acting wash over you.