I (rogue) do 2s with a priest and we faced a demo lock and unholy dk the other day. I could not kill the lock in demon form before my priest died. All those pets were ridiculous. Demon form lasts for so long you can't really kill them. They're susceptible to cc's but if I blind either one to try to save my priest, they'll trinket it easily. The lock didn't get in range for psychic scream and so I'm stuck sitting on him during his demon form to prevent fears while the dk beats the crap out of my priest. I try to disarm the dk here and there but it's still a tough ass fight.
In world pvp, demon form might not be hot as you'll be taken down before you pop it but in arenas, it's really good. I can't kill you in any timely fashion.