I've been playing Demon/Dark for a while now and I'd say Hell on Earth is very worth it. First off it acts as a 90 second Accuracy & Damage buff for the pet you use it on. Also it will periodically summon extra minions doing 50+ dmg a hit each. Its long cooldown means you should probably save it for the bigger fights.
I recommend enhancing it with expedient reinforcement, and taking haste as part of your build. Typically I will cast HoE on my Demon Prince before a big fight, then cast haste. Doing that I'll be able to recast HoE just a few minutes after the last cast of HoE ended.
