Awful. Terrible. I hate saying that because this came highly recommended by a few people...but it's true.
First off, I did not finish this. It was a struggle to make it through to the third chapter, which is where I finally threw it down and yelled some bad words.
The author's note at the beginning is the first thing that made me think maybe this wasn't the book for me. I really, really dislike it when authors try and treat readers like they are so dumb that they will accept that this book was the product of (insert fictional character here)'s actual writings.
Mary Russell is apparently spelled M-a-r-y-S-u-e according to King. She is good at everything, except when it's convenient for her not to be good at something for a nanosecond, and then that's quickly rectified to explain that she's just the most wonderful and intelligent person, barely even second to Holmes himself. She enjoys a huge amount of personal freedom inconsistent with the time and declares every few pages that she is a FEMINIST and we all know that feminists always disguise themselves at boys to prove their feminist street cred. Now, I don't have any problems with smart female characters...I usually love them. But Mary doesn't come across as smart, just super annoying.
Holmes is a tame, warm and fuzzy sort of character who gets screamed at by Mary in the first few pages and just seems to be oh so charmed by this that he invites her for lunch. And apparently he's just been waiting for this exact person to come across him and now his life is complete with Mary in it. She's even so fantastic for him that he gives up any and all drugs. AMAZING.
...which brings me to what I assume happens later in the book or series (confirmed by other reviews, and Wikipedia), which is that this 15 year old girl and 50 something man get married. As my own significant other is 7 years older than me, I shouldn't have any problems with a May-December romance, but a) this age gap seems a little extreme (39 years), and b) the romance aspect is probably the whole goal of the book series in King's eyes. Was there something wrong with them being partners in solving crime without a hint of romance? Nope, we gotta have romance be the reason for everything. Always. Everywhere.
And last...the depiction of Watson as a bumbling fool. No. Just no. Conan Doyle makes it perfectly clear (and this is consistent except in a few of the adaptations I've seen aka Rathbone) that he brings the humanity to the partnership with Holmes, and that Watson is incredibly important to him. They are a team, and this dismissal in favor of a Holmes-clone, regardless of gender, is incredibly disrespectful. Holmes doesn't need someone who is his equal in intelligence (although Watson isn't dumb). He needs someone who complements him. Watson did that, and that is why he is important.