In California we use it all the time.  We also use wanna and gonna.  I wouldn't use it in written form, but as far as speaking you can definitely use it.  It adds color to your conversation and can be used for emphasis.

There are two different ways to speak English:  The first way is to pass tests and to get correct answers.  The second way is to get people to like you, be funny and interesting in your conversation, and have a way with words that makes people want to listen to you.

As long as you understand that both can be important, use them how you choose.  

In older times we used to have words that are now never used, such as mayn't, amn't, an't, which are all contractions as well. Just like ain't, they have been used in literature, theatre, poetry, journalism etc., for hundreds of years.

Except for ain't, they have all fallen out of use to such an extent that most people consider them wrong. And yet, some of the English speaking world's greatest writers have used them repeatedly in their works.

It's true that ain't is usually restricted to specific informal settings and socio-economic groups, but in my opinion that doesn't make it any less valid. If my students use ain't, I explain the usage, and when to use it, but I certainly do not tell them to stop using it. 

I think just about every native speaker says "wanna" and "gonna", but a lot of them don't even realise they're doing it. About a year ago someone asked about them and I mentioned that I personally never used them, and on the same day I wrote that, I 'discovered' myself saying them without meaning to. I honestly thought I never used them, but I only noticed I use them all the time after writing that I didn't!

"Ain't" is different for two reasons: a) it's a completely seperate word (you can't 'accidentally' say "ain't" by speaking quickly, the same you can accidentally say "wanna" when saying "want to" quickly), and b) "ain't" isn't nearly as widespread as "wanna" and "gonna". According to my own survey, 9% of native speakers say they use "ain't": https://www.surveymonkey.com/results/SM-MSL77QHC/

I'd recommend you skip using the word "ain't".  Non-word or not, I wouldn't use it.

No, it ain't !

In fact, "ain't" has been around as a meaningful element of non-standard grammar for a long time. It's a non-standard version of 'isn't' and similar negative contractions. 

"Gonna" and so on are non-words because they are simply transcriptions of speech sounds.