For Demonology you generally want to stack it with Metamorphosis (use both at the same time where possible). Not sure about other specs - Demonology would be using a Felguard or a Succubus which means the use is basically as a DPS cooldown to be used as you would use any other cooldown - when you need burst damage, same time as bloodlust, etc.

Looking at the spell tooltip - I would imagine that Destro (using an imp) would use it as a DPS cooldown, and that warlocks using a felhunter would use it at the start of a fight applying dots, and then perhaps when dots needed to be re-applied if this coincided with it coming off cooldown (not too sure about this).

As a destro lock you want to use it on cooldown. The only exception is on boss fights where you want to wait until you get hero, pop demon soul and watch your dps meter go through the roof.

as demonology you want to use a FG on aoe fights, or an imp in single target fights. the imps demonic empowerment gives it the edge over the succubus, and in simulations, it gives better dps.

Herman, I love you. So sick of hearing succubus noises while I raid. I'm definitely running on slightly old data.