It is quite a serious buff, 13% extra damage (with soul link) if you're only spamming shadowbolts in raids, + you get to use the succubus for some additional damage and seduce cc. The other buffs are pretty good too, 2%mana/3%health reg per 4 seconds means less tapping and happier healers.
Would really like a GM's response on this to see if we can use it or not. I personally won't use it until a GM says I can.
The patchnotes that Nervous is talking about can be found here:
http://www.wowwiki.com/Patch_1.12.0- Code: Select all
The trick of Demonology specced Warlock Engineers using Jumper Cables to resurrect a pet that's been Demonic Sacrificed, thereby giving both buffs, has been removed. The buff from Demonic Sacrifice now vanishes whenever you have a pet active regardless of how that pet got (back) into the world.