4 - Before the abominable aberration that was Season 4 (whose reversal of this point was its single positive quality), there were three things in life that were completely inescapable: death, taxes (unless you're rich), and the soul-crushing mediocrity of any given mid-season Sherlock episode.
I have to agree with Zachary below, more than anything this episode reminded me of a BBC update of "The X-Files" - what with our two mismatched investigators, dressed in black and investigating a seemingly paranormal occurrence in the countryside. Overall, it's one of the weaker Sherlock episodes since it really does just follow a 'monster of the week' formula. Still, an elegant time-waster.
It's pretty ludicrous and absurd, and unlike The Blind Banker which held a tighter reign on the absurdity, The Hounds of Baskerville is almost like an episode of The X-Files. But, boy is it a lot of fun. Government conspiracies, genetic engineering, monsters and mayhem! I love it!