Despite being born and raised just outside of Atlanta and spending almost 30 years there, I don't use the word "y'all."
Chalk it up to my Yankee mother or the fact that the mix of accents in my life resulted in my being in speech therapy throughout elementary because people couldn't completely understand me sometimes, but I never adopted "y'all" into my vocabulary. It can grate on my ears, and I end up saying "you all" instead. (It could be because, like "bless your heart," "y'all" can carry a lot of meanings depending on the speaker's inflection, and I'd rather avoid embedded connotations.)
My personal dislike for it aside, "y'all" has a useful and important place in American English as it solves a sticky issue in the language, namely the lack of a plural pronoun. And the solution it presents is making "y'all" more and more common around the U.S. Even my partner, who was born and raised in Southern California, uses it all the time.
Where'd 'y'all' come from?
In American English, we lack a second person plural pronoun. "You" is the second person pronoun, but we don't have a plural form of it, so it just gets used as the second person plural pronoun, too. So, if you're talking to a singular person, you'd say "You" but if you're talking to a large group, well, you still use "you." It can be a touch confusing. Say you walk into a room with one person, and ask, "Can you hand me that pen?" That lone person knows who you're talking to. If you do the same thing and there are multiple people near the pen, no one knows which "you" you're addressing because you're addressing everyone but also only one person because of how "you" functions.
Confusing, but only in English. Spanish speakers will use "vosotros" or "ustedes" (the former is more informal while the latter is more courteous) while Germans have "ihr" to function as second person plural pronouns. English used to have second person plural pronouns a long time ago, back when "thou" and "ye" were tossed around. "Thou" was a singular second person pronoun while "ye" was the plural form. Regrettably, we lost the use of both (until you visit a renaissance fair), by and large, as French began to influence Middle English following the Norman invasion.
"Ye" stuck around, sort of. It morphed into "you," which was one of the "ye" conjunctions in Middle English, so it made sense that we'd eventually end up using it for both pronouns. And here we come to the fact that scholars aren't completely sure where "y'all came from. According to Salon, there's debate about whether or not the contraction of "you all" came from the Scot-Irish "ye aw" or from early African-American vernaculars or an African-English creole that used "you all" early in the 19th century. It's also possible, even likely, that it was a confluence of both traditions sweeping across the Southeast and Appalachia at the same time and the end result was "y'all."
'Y'all' isn't the only solution, but it might be the best one
Plenty of regions in the U.S. developed workarounds for the lack of a second person plural pronoun. "Yinz" is popular in Pittsburgh and other parts of Western Pennsylvania, while a little further north, in New York, you might hear "youse," a literal pluralization of "you." Both are also kinds of amalgamation of "you ones" or "yous ones." Like "y'all," both are markers of their regions, in many cases immediately identifying the speaker's likely home.
Neither of those other versions of the second person plural really work outside their region, however. You have to explain "yinz" outside of Pittsburgh, and "youse" can be too easy to slip into like you're in an Edward G. Robinson film. And while "y'all" has all its southern connotations, it's also a clear contraction of "you all," perhaps giving it a bit more credibility outside the South.
"Y'all" is also a better alternative to "you guys," which is perhaps the most common plural form of "you" American English uses. It's too casual for many instances (though I feel "y'all" is also very casual, but that could just be me). More importantly, it's also gendered since "guy" generally refers to someone who identifies as male. As we become increasingly aware of our pronoun usage and how gendered language is a form of transphobia and sexism, "y'all" encompasses all without gendering the subjects.
While I may stick to "you all" despite its general clunkiness — "y'all" does roll off the tongue very nicely, and it does solve a sticky problem in "proper" American English.
Whatever you do, though, don't spell it "ya'll." That's just wrong.