In an attempt to come up with a consistent ruling on Iron Will (since different books have different descriptions of the merit), the following will go into effect as of this posting:

Iron Will – This Merit allows you to resist the effects of vampiric Domination, Mind magick or other coercive mental attacks. The Trait does nothing to deflect mental trauma or deception, but makes it damned hard for someone to mind-control you. Potent attempts from really powerful foes might demand a sacrifice of a Willpower point per turn.

In game terms, this means if a character has Iron Will 3, he/she will be immune to mental attacks from a character with a power rating at 3 and under in the mental power (i.e.; Mind 3, Rank 3 gifts, Dominate 3, etc.), and those will Iron Will 4 will be immune to mental attacks from those with a power rating at 4 and under. If a character with a power rating OVER your Iron Will rating attempts to use a mental power against your character, willpower will have to be expended to resist.

Both Supers and Mortals will use the same version of Iron Will, and both the 3 and 4 point versions are available to both. Those with Iron Will 3 as of IRL September 21, 1998, will be able to upgrade from 3 to 4 for a cost of 2 XP. Anyone gaining Iron Will after that date will have to have IC reason to upgrade from 3 to 4 as well as the XP.