It's for Allah willing , and yes it's a common in everyday conversation .
For me, it's not sound that good as a modal verb because it's not a verb or doesn't have anything related to tenses. It's just an expression Muslims use it to express the idea that if Allah wanted us to do something we'd do it, but if I promised to do something and circumstances were against me and my will, then it'd not be my fault to not having that thing done.
And yes, you can use it with both negative and positive form. But usually, people say insha Allah as an answer for no in some cases, but that meaning wouldn't be explicit. If you asked me to do your homework for you and I said , yea , insha Allah. Iit'd mean no.
It's more an Islamic expression than being Arabic one, so it's up to you whether to use it or not. But for me, when natives use that expression, they sound friendlier somehow.