I never got the doomguard pet, either, because it is so messed up. Guaranteed to kill 1 party member at random, and it could be you which means you just died and chances are another party member or two will die as well by the hands of the big demon you just summoned. I have summoned doomguards from curse of doom several times but, again, they appear at the end of a battle where you're likely to be injured or low on mana, you have to enslave them (and good chance it gets resisted, or you waste time debuffing his shadow resist), and then it's nearly guaranteed to break at the worst possible moment. Blearg.
The infernal has definitely seen some use, though. I've used it in pve situations to clear out campsites of mobs, and I've used it in pvp against large groups of allies. Like the others said, the aoe stun is nice, and it has a good damage shield. The bad part is once you use it, you can't summon another one for 1 hour. Oh well, it was fun while it lasted, right? :)
Optamos