I think people are really too hard on these books! If it was a guy character who got trained by Sherlock people wouldn't scream Mary Sue. It's only fandom that's trained to think any female characters with amazing powers of whatever (like Holmes, except a girl in this case) is SO WRONG.

This story made it onto the list of the hundred best mysteries of the twentieth century. It's extremely well written, and, at least from my person POV, Watson is largely removed from the story (as he was in some periods of HOlmes life) but not treated horribly or made into somebody to bash or belittle.

There's no point defending it; if someone doesn't like it, they needn't. It just think it's rather short-sighted and foolish to bash on this story for these reasons - really not good ones IMO.