I'm ridiculously pedantic when it comes to Sherlock Holmes, for obvious reasons. I never much like pastiches, either. My theory is, why can't we just leave a good thing alone?
That being said, this is an excellent Holmes and Watson pastiche. Carr has a good feel for Doyle's narrative style and (this may be what sold me on the book) we get to see much more of Mycroft, who makes all too short of an appearance in the original Holmes collection. Importantly, Carr seems just as true to Watson as he is to Holmes--he is portrayed as the intelligent, staunchly loyal medical man that he is.
The book also incorporates a lot of action (those who yearn for Victorian train travel and political espionage will not be disappointed) and, although a good read, this is when the reader is most reminded that it is indeed a pastiche. However, the book is no less enjoyable for that.