Note that this post contains many possible spoiler links for season 2. Read at your own risk! And... you will very probably cry if you read many of these.
I'm not Johnlocked. Really, I'm not. But my friends who are will offer me their favorite fics to read, and I read them: I wept over Alone in the Water (which is very well done, and loaded with angst), and Sink Like a Stone is another story of high caliber and great pain. Bring Kleenex along for them both!
Why do I read the Johnlock they suggest? I'm a voracious reader. Not read something?? Only if I really don't have any interest whatsoever -- and there is some writing that fits that -- while fic about Sherlock and John, two characters I adore... of course I'll try it. :D
And I've read Present Danger
Beautiful Dreamer is gorgeous. I had to keep playing Coldplay's Fix You constantly as I read it, in fact.
The other fic by the same author, What Meets the Eye, is also gorgeous. These are the only Johnlock stories I've read that make me able to see how the underlying premise could really fit the characters.
I've read WMtE twice, two nights running. Because it's that good. Makes me cry. Thanks to Gita for sending me to BD and Nika to WMtE!
Some of my favorite Sherlock fan fic, which is not Johnlock:
The Holmes Boys: Epiphany #1 Try #2 Philosopher #3.
SPOILER ZONE: The Dog in the Night (post-Reichenbach).
So consider this a primer into what Sherlock and Johnlock fan fic can be like, if you would. :D
ETA in March 2013: The only Mystrade I've read, which is really excellent and has hooked a few Tweeps I know on fan fic. :D It's a series. Do go try it!