I've read more Harry Potter fanfiction than I have fanfiction of other genres. That being said, I've found HP fanfics to be either wonderful pieces of literature, or aweful dregs that should never have been written.
I've been a fairly loyal fan of the SSHG pairing for many years now. Lately, though, and I don't know why, I've been getting to dislike it more and more. I think it's because after you read so many stories in a certain genre, you've read them all. So, recently, I've been reading chapters of stories I've already started in that genre, but not starting any new ones. I've been exploring a bit. To tell you the truth, exploring scares me. While SSHG stories may not be the most cannon (I accept that) I've found them to be some of the better written.
A few things in fanfiction bother me. One thing is the insertion of new characters. I know it's inevitable, but if a new character doesn't strike me as interesting right away, I'll walk away from a story right there. I also really dislike it when characters from Harry's time go back in time to when James and Co. were in school. I have really only liked one story where that took place, and it wasn't the generic, "Oh hi! I'm here and everyone accepts me!" bs. that is normally present in those stories.
I believe I've read well over 100 fanfics in the HP genre. I do have a few favorites. One, I can't link because it's rated R. It's about a dementor, specifically the one on the train in Harry's 3d year. It's very cute, and funny, and written in almost exactly Jo's style. In fact, were parts of that story to get out on the internet, it may well be mistaken for the 7th book. Well, except for the part in which the Dementors reenact scenes from Les Miserables...