Finally saw this. It was cute. I'm not familiar with this book series, but based on the trailer this was very much what I thought it would be. If there are more made I will probably watch them. The plot was definitely dodging some holes, but it moved briskly enough and was sufficiently charming to keep me engaged. I agree it could have been shorter.
Millie Bobby Brown did very well as lead, and I liked seeing the character stumble as much as she succeeded. I was hoping to see her set up at Baker Street by the end, but it worked out okay. I rolled my eyes a little at the teen romance, but there was respect on both sides and they both had their good points.
This is probably more in keeping with having two sons like Mycroft and Sherlock. I thought they were setting up Mycroft as the villain, but the grandmother was interesting. Mycroft is instead the proper bureaucrat. I thought it was odd that they referred to him as less gifted than Sherlock since in the Doyle canon that isn't the case. Another notable change is the absence of a Watson in Sherlock's life. Traditionally Watson is the window into Sherlock that lets the reader/viewer see his human side. It seems Enola will play that role here, to the extent that Sherlock will appear.
I thought Henry Cavill was fine as Sherlock. He's a supporting character/mentor here, so I didn't expect great emotions or revelations from him. He was expressive when the moment called for it. He was positively warm compared to some versions of the character. This universe puts him in the position of not just younger brother but middle child, and that's an interesting dynamic. I agree the scene with Susan Wokoma was good, and I liked the laugh of surprise and delight after his conversation with Lestrade.