Having watched one of the movies but never read the books, I can now see why they chose Robert Downey Junior as Sherlock. Perfect choice. Downey pulls off Sherlock's arrogance and self assurance to a T. 
 As for the book; the mystery was entertaining, Sherlock's deductions are so precise they border on the unbelievable. I appreciated Doyle introducing a back story for the murderer and going off on a tangent, it worked well. The ending was a bit illogical and unreasonable (view spoiler)[ Hope already knew Holmes address as he had sent his friend to reclaim his ring. Therefore, why on earth would he go back there when someone asked for him by name? Specifically since he mentioned fearing that the ring's address was a trap. The only reason he'd go was if he wanted to get caught, which he clearly didn't want, judging by how hard he fought. (hide spoiler)] 
 Besides the illogical moment, I'm also giving this a 3 because we don't know anything about the characters. Yes, we know that Watson was in Afghanistan and that he's a doctor, but other than the odd bits here and there, the characters were very shallow.