Anyway, here we go. The Doomguard is the last demon we'll discuss on Demonology 101. Lazy Warlocks such as myself can luck out on a Doomguard with Curse of Doom, our favorite spell on Netherspite. It's a fairly low percentage to spawn one, and Curse of Doom needs to deliver the killing blow on the mob... which can be a long wait considering the curse only deals damage at the end of one minute.
[UPDATE: Curse of Doom has been revised to spawn a Doomguard 100% of the time if it delivers the killing blow! Ritual of Doom also no longer kills a player but damages them, apparently with a Curse of Doom-size damage spell. Doomguards remain enslaved for 15 minutes and disappear thereafter, eliminating the need to enslave.]
As you can see, it's not a particularly reliable or even sensible spell to begin with. On the other hand, Ritual of Doom is guaranteed to spawn a Doomguard, but it requires a Demonic Figurine and five people to complete. Oh, and one of the five people will die. It can even be you, the Warlock, so be sure to have Soulstone up. It's not an easy spell to cast, by any means, but the good news is that the Doomguard is a way better pet than the terribad Infernal. Just remember that if you do spawn one, it will be hostile towards you and will need to be enslaved.
It has great abilities like Rain of Fire, War Stomp, Cripple, and Dispel Magic. All of these abilities are fantastic for PvP except that it's complicated to bring a Doomguard into play, specially since Curse of Doom won't work on players. And even if it did, good luck on getting a one minute dispellable time bomb to deliver the killing blow. The Doomguard is easily the Warlock's most powerful pet, despite the fact that it doesn't benefit directly from any talents (similar to the Infernal) except for Improved Enslave Demon. In Wrath of the Lich King or Patch 3.0, the Doomguard will get more health, more mana, and... that's it. Eyonix is supposed to have mentioned that Blizzard would do something to make the Doomguard more useful but right now all we get is more health and mana.
The Doomguard is a great pet. It's fun to use -- the AoE stun on a 20 second timer should be well worth the price of admission (the AoE stun is the best thing about the Infernal, too). Unfortunately, it's simply too difficult to bring into play. You can curse gray mobs (it can still spawn a Doomguard) and wait for it to proc. But since it takes a full minute to work -- the spell's one minute cooldown prevents cursing multiple mobs -- that might take a while. So until Warlocks get a more reliable way to summon a Doomguard, Ritual of Doom only works best as the Azerothian version of Russian Roulette.