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'm the opposite, I never say y'all on the regs, unless it's for jokes. No one ever believes I'm from the south. Except at work, I like to talk like a southern belle to guests, bwahahahhahahhaha, it makes me sound very friendly, because if I talk how I want to talk I sound like april from parks and recs.

I would definitely say, it's informal, I've never heard of it being formal, ever.