This is a great spell for warlocks in arena. With all the curses a warlock has, a priest will be forced to dispel, which results in the direct damage.
In PvE, this spell is just another DoT. Most raid bosses do not dispel, and therefore the damage and silence of effects of dispelling this spell are wasted. However, affliction raid specs should still use this spell in raid settings, since its damage done per second casting is still higher than [Shadow Bolt].
In PvP, it can help prevent your DoTs from being dispelled safely, as its secondary effects can cause significant damage to the dispeller.
Patch 3.2.0 (04-Aug-2009):Immolate and Unstable Affliction: All ranks of these two spells will now overwrite each other.
/Patch 3.0.2 (14-Oct-2008): Silence effect reduced to 3 seconds, down from 5.
Patch 2.3.0 (13-Nov-2007): The silence from this ability is now subject to diminishing returns.