Theralion's Mirror seems like an extremely effective trinket for demonology.
Normal version gives 321 int and a chance on cast for 1926 mastery rating, while heroic version gives 363 int and chance for 2178 mastery rating.
Demo's mastery is demon form damage and pet damage and the mastery that you get from the trinket proc is about equal to 19 points of mastery. With 2% increased damage per point of mastery your looking at an addition 38% damage from pets and demon form when this procs.
So if your already have about 19 points of mastery when you go in demon form you get 58% increased damage + 10% more from demon soul + whatever your racial profs give + another 38% from mirror proc if you wait for that before you use demon :P thats over 100% increased damage and thats just insanity.
Do you all think that mirror is bis for demo?