I'm currently updating my Blizzard launcher on my 7-year-old laptop because it turns out Demon Hunters are going to be live two days before I get back from a trip home, and if I can, I'll get my super-creaky old computer in a place where I can roll up my Demon Hunter and play through the starting experience at least (I don't even have a mouse here, but I've been meaning to get a new one as the old one is ten years old and the rubber on it is largely worn away.)
Anyway, on August 9th the Burning Legion invasions will begin. The Broken Shore event, with its follow-up quests, will presumably also unlock at the same time. And players who have pre-purchased Legion will be able to create their Demon Hunter characters.
Demon Hunters will not be able to do the Broken Shore event, given that it's the aftermath of that event that sort of brings the Illidari back into the fold. However, as we fight across Westfall, Dun Morogh, Hillsbrad, Tanaris, Northern Barrens, and Azshara, we'll have some horned, tattooed, eye-gouged elves fighting alongside us.
Demon Hunters will probably not be as powerful at level 100 as other classes, as they'll unlock most of their talents on the path to 110, while established classes will have all their talents already available to them. Still, given their special strength against demons, they shouldn't be at too much of a disadvantage when dealing with these invasions.
I am pretty darn excited - this is less than two weeks away.