Mixing on headphones is a mixed bag (lol)… Some people swear by it, some really don’t. I don’t like to mix on headphones personally, but good headphones are still an essential tool to have in anybody’s arsenal.
And no, your DJ headphones won’t do, no matter how expensive. Those are optimized for an entirely different thing – reproducing things (particularly drums) within loud enviroments. DJ headphones are far from ideal in studio enviroment because of their closed structure.
Good headphones will enable you to “zoom in” really close on your sounds and mixdowns and hear things in great detail. This can be a crucial thing especially if your monitor speakers leave someting to desire for or if your room acoustics are a bit funky.