Really good watch, might watch again, and can recommend.
To absolutely fair, I'm not sure if this will appeal to Sherlock Holmes fans, but does expand the "Sherlock-verse", so make up your own mind.
I love Millie Bobby Brown ("Stranger Things"), and this proves she is quite capable of not only leading a movie, but carrying the entire bloody thing.
Despite what feels almost a lack of contribution from everyone else in the movie, this actually turns out quite good. I do wish she didn't do the fourth wall breaks, it really feels like she's either turning to a documentary crew or suddenly updating instagram. It was a little jarring, and they're clearly referencing Sherlock's "mind palace" techniques displayed in the movies, but with a "twist".
I honestly expected someone to ask her if she's okay, like in "Dora and the Lost City of Gold". It would have been equally acceptable to do narration over her actions in the same, but I understand wanting to do something different.
The only other people that didn't seem to be phoning it in were Burn Gorman who did quite a good antagonist, and Frances de la Tour who managed to elevate "concerned grannie".
Maybe Sam Claflin did the job perfectly and Mycroft is supposed to be that bland and irritating, but Henry Cavill had an air of being relieved that didn't have to have Sherlock doing any real deductions, he just had to stand there and seem mildly intrigued by things.
Overall, there is a good movie here in spite of itself, and the sad part is that it could have been great.