I've been messing around with it for fun sense they nerfed ua. I've had my felpuppy crit over 40k on targets with resil, Demo has great burst damage but not high sustained damage. Pet damage is great but felpuppy is way easier to cc than felguard.
What I have been trying though is using soulburns to switch between felguard and felpuppy.
I have a simple macro thats
/cast Spell Lock
/cast Axe Toss
saves me a keybind and does either silence or stun depending on my pet.
It ends up giving me a great amount of cc if I time it right with hog deathcoil fear leap and howl.
I think its good against certain teams however demo has even less self heals than afflict and when your out of demon form if your unable to shadowflame and run or demonic circle away you can get exploded so quickly even with soul link and almost 4k resil