In general, Horowitz captures Watson's plodding literal style of narrative, and the odd blunder doesn't jar too much. On the other hand, the story has a modern feel to it, with its concern for the underprivileged, and the purpose of the House of Silk. I didn't think that Ross's death needed to be so horrible.
The book is full of improbabilities, but so are all of the original stories. In fact most of the Conan Doyle Holmes stories have major weaknesses, including some of the best known ones such as the Red-headed League and the Speckled Band. The long stories, with the outstanding exception of the Hound of the Baskervilles, are particularly poorly constructed, with the present-day action being understandable only in the light of events which had taken place many years before.
Overall it's not a bad effort. The main weaknesses: the Moriarty incident - quite superfluous to the plot; the clue about the (spoiler alert) House of SILC (silc) (see what I did there), flattering the reader into thinking he has got there way before The Master (I kept expecting it to turn out to be another red herring, but no); and the length of the novel, which threatened at times to slow down the fast pace of the book. And Holmes isn't quite how I remember him - he seems to have acquired a 21st century social conscience. I liked the old selfish bastard Holmes better.