First, I consider AU and OOC to be different types of fiction. When I read an AU fic with Brian and Justin, I want to still feel like I'm dealing with the same characters as the show but in a different setting. If well written, then even if the story takes place in 19th century England, Brian would still act, sound and feel like himself within the structure of this story.

I don't enjoy OOC stories because then the men might just as well be named Bob or Joe because they are no longer recognizable as Brian Kinney or Justin Taylor.

Now, why I read them. I am grateful to say that I've read some really good canon fics in every part of each season. But honestly, how many post513 stories can you read without getting bored? I found that after some really good ones (like plumsuede's BYBR), I just didn't want to read any more in this time frame.

A current story that exemplifies an AU but still very much IC, is Timeless by cynical21. It's mostly AU but she keeps the characters very canon-believable (to me).