In general, mages it is good on tend to be those who are more damage over time than bursty, (who often unload their burst before getting hurt), and who have shorter ranges or have to be in the middle of the team to do damage.
So its pretty good on Karthus and fiddle, who tend to dance in the middle of the enemy team to use their deadly auras. Good for Ryze who has to get up close and stay there a while with the enemy in a Force cage (shooting him back if ranged).
Classic bursty mages like Annie tend to go in, unload every spell they have, then run out (under shield if they are annie), making it much less useful.
Its also common to build hextech revolver for the laning phase, and upgrade later when the aura is of use as you have teammates around.
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jadelink ?? Senior Member