From a boxing point of view, Demonology is by far the easiest spec.
If you're single boxing, it will likely be the worst spec, but with pet enhancements in 3.2, it might gain some of its effectiveness back.
In BC, for Tier 4 and Tier 5 content, Destruction could not touch an equally geared Demo lock.
By Tier 6, Demo was still better then any other class for DPS, but Destruction was better.

Single boxing, Affliction is consistently higher then Demonology in my guilds raids.
Still Demonology feels more like warlock play to me, then Destruction or Affliction.

If its also working well for you, and will have significant buffs next patch...
Good to hear.