"The Hounds of Baskerville" delves into the Sherlock/Watson relationship perhaps more than any other episode of this BBC series to date, so let's take a minute to discuss why, exactly, John Watson puts up with Sherlock.

The question of why Sherlock needs John is pretty obvious. From a practical standpoint, John is his sounding board, he'll do footwork for him and he can interact with normal humans in a way Sherlock can't.

Plus, he's a crack shot capable of physically defending Sherlock, as in the very first episode. Beyond that, Sherlock clearly values his friendship, even though he's loathe to admit it.

For John, there's the simple answer: partnering with Sherlock helps give him the same rush and excitement he got from the war, and also helps him avoid dealing with his PTSD.

But there are plenty of soldiers who deal with PTSD in ways that don't make them codependent on a condescending sociopath who misleads, insults and drives away their girlfriends as a matter of course. Oh, and how tries to give them hallucinogenic coffee.

Beyond that, I think John has a surgeon's stubbornness and arrogance in his (accurate) belief that Sherlock needs him, and that he can, if not change Sherlock for the better, at least curb his worst qualities. (We get a hint of some of this same quality in the mentions of his relationship with his alcoholic sister.)

Other thoughts:

• I like that John's offense at Sherlock trying to drug him was overwhelmed by his delight that Sherlock was wrong.
• I thought the UMQRA Morse code bit was perhaps a red herring too far in an episode full of red herrings. The payoff wasn't really funny enough to justify it, making it feel like padding.
• There was almost no Mycroft in this episode, despite being (or perhaps because it was) written by show co-runner Mark Gatiss, who also plays Mycroft.
• This episode gives us the famous Sherlock Holmes quote: "Once you've ruled out the impossible, whatever remains, however improbable, must
  be true." As IMDB notes, Watson calls Sherlock "Spock" shortly after, and Spock said the same thing in "Star Trek II," crediting the quote to an ancestor. I will threaten to break the space/time continuum by pointing out that Benedict Cumberbatch has a role in the sequel to the recent Star Trek reboot.
• The show has tremendous art direction and attention to detail, and I particularly like Watson's military-inspired jackets.