I presume j'suis is slang for je suis.
That much for the lexical formation of this "expression," which has been detailed elsewhere.
However, I don't know the contexts in which it can be used. Is it considered an offensive way of speaking, a derogatory way of speaking, or a perfectly harmless way of speaking between friends and family?
Can children tell their mothers, in a spoilt brat manner, J'suis?
So what I'd actually want to know is a few details about its usage in the language nowadays.