It's a standard result that the continuous locus is always G-delta$G_\delta$. For each r>0$r>0$, let U(r)$U(r)$ be the set of points x$x$ such that some neighborhood of x$x$ maps into some ball of radius r$r$. Then each U(r)$U(r)$ is open, and the continuous locus is their intersection. Conversely, given a G-delta$G_\delta$ set, I'm pretty sure it's not hard to construct a function with that continuous locus, though I don't remember how off the top of my head.