Haven't played a DH myself yet, but they're everywhere (obviously) and quite powerful at the moment. I have taken a few on solo in PvP, not sure which spec they were in, but others melt me quicker than I can self heal (which is often for a Blood DK). In group PvP their AoE is insane, which also leads to them being top dealers in dungeons due to lots of trash. Their mobility in PvP is insane, they can double jump and glide, the can do a blink like dash, and when fighting at melee some of them jump all over the place and are very difficult to follow. On top of that, I think most DH's who levelled quickly picked Leatherworking due to the ridiculous amount of leather you got from Skinning (which has now been nerfed hard, but before the nerf I made almost 90k in leather just from levelling skinning 1-800) which helped boost their gear, and therefore power, very early. Whilst most in my guild were starting mythics (around 815-820 avg ilvl), one DH had 850+ gear in every almost slot almost exclusively from LW.
I would imagine some "balancing" incoming which will no doubt lead to some of them going back to their old main characters, much like when DK's where released. But I hear their story is very good and the starting 98-100 zone, like the DK's, is instanced and very well laid out. However, be prepared (pun intended) to hear every DH in the world tell you over, and over, how much they have sacrificed...