I'm the queen of y'all, I use it WAY too much. That being said, it's most definitely informal and I'd never use it in a formal manner of writing.
Originally Posted by spring1onu
I was just coming back to say that. I hear it in informal or day to day conversations, everyday, but I have never seen anyone use it in formal writing. When we were in 1st and 2nd grade, some tried, but they learnt better

Yunz is also acceptable. Lol
When I hear terms like "hipster" I think, who told cliques they could leave high school??