This is a fun and entertaining read for any Holmes fan. Horowitz has largely managed to duplicate Watson's voice (apart from a few slippages e.g. he talks about Holmes' `metabolism') but this is a Watson who is far more emotionally needy, even fawning and sycophantic, than Conan Doyle's doctor. The homosocial, if not homoerotic, overtones to the relationship, in line with other modern reinterpretations, is foregrounded here, and the plot starts off very well.
However, the authenticity is soon derailed: Watson and Holmes suddenly turn into Dickensian characters appalled by the divide between the wealthy and the poor, and shocked by the plight of children on the street. Holmes regrets ever having created the `Bow Street Irregulars' and berates himself for not having enquired more into their terrible lives. Even the relationship between Holmes and Lestrade becomes something quite different from the original.
This isn't trying to be anything more than light entertainment so it's probably not fair to be too picky - I liked this well enough but I'm not sure that I'd bother with a sequel.