I work with someone who uses them and I find it impossible to read his screen. Unless you work completely alone, it's a terrible idea.
If you work with people over the internet, it doesn't matter because they can disable ligatures and still read your code. And obviously it's not a problem if people know them and are looking on your screen.
But yeah, if you're doing pair programming and first dev is used to them and the second dev is not, that's a problem. The question is, is the solution that the second dev learns them or that the first dev stops using them - both will obviously dislike changing the way they like to code...