I'm not one of those readers who thinks that anything that veers from the "accepted" facts about Sherlock Holmes and Dr. Watson in a post Conan Doyle Holmes novel should be treated as heresy and accordingly ignored at all costs. Granted, when Holmes starts fighting Martians I can get a little frustrated (and bored) but I'm more than open to a little "re-imagining" if it is well written and interesting.
The Veiled Detective is such a book; in it's own way it asks for a lot of suspension of disbelief (which may be a strange thing to ask about a fictional character anyway) with the clever way it introduces Holmes and Watson, the part played in this by Professor Moriarty and yet another hidden secret in the form of Mrs Hudson's motives. I have no doubt that some readers will be up in arms long before the climax wondering why Davies had to meddle.
That said most of what occurs is, in it's own fanstastical way, very believable and for the most part is logical. Even the nagging feeling I had whilst reading the book (mainly that Holmes seems blind to the position that Dr. Watson, who is not what he seems, has been placed in) is explained away in a satisfactory manner towards the denouement.
If ultimately it doesn't quite convince us (mainly becuase we are so fond of the Conan Doyle versions of the characters) that doesn't mean that it isn't a very entertaining read and if you don't mind the obvious liberties it takes with the accepted backstory I'm sure you'll enjoy reading it.
Oh and like a previous reviewer, certain moments in this book also reminded me of the recent Benerdict Cumberbatch BBC Holmes production. Either Davies was involved with that or someone close to the production had read this book.