I know a few people who use haXe professionally.
One of them that I know personally is a professional Flash developer who used haXe exclusively for some times, because it had a better standard library than Flash (AS2 at this time) and generated lighter and faster SWF files. I remember him telling me that he made a somewhat huge (several hundred euros) donation to the project and that it still cost him less than buying the Macromedia Flash software. Since AS3 he's back to Flash thought.
Some others I know through their work: Motion Twin [1] for instance, who are the creators of a lot of very successful Flash games.