Overall, my sense of the K/S relationship is that the axes are different, as mentioned above, and who-plays-what-role switches around a little. One is not so much a sound-board or foil or "genius wrangler" for the other so much as both are necessary to one another.

I agree. Spock is a weirdo by human standards, but he does all right by Vulcan ones and is constantly citing the same; despite McCoy's computer jokes he doesn't come off as inhuman the way Watson sometimes portrays Holmes--the show goes out of its way to show him having emotions underneath, which is frequently an open question in Holmes book-canon.

I guess in some ways the net effect is the same--they need each other--but how the relationship works is not quite the same. Kirk says "Risk is our business" and Spock doesn't disagree; Watson would at least have reservations, I think.

If melannen gets to cite Gilgamesh/Enkidu, I'm going to throw in Achilles/Patroclus and Alexander/Hephaistion. Which again in both cases you have the half-divine hero ruinously attached to the seemingly unremarkable guy.