I agree with what's been said here that the DM needs to define some limitations of PF in terms of what it can do and how people interact with it. I like the saving throw bonuses based on missing sensory input. I also allow targets a saving throw to disbelieve the illusion if they interact with it or see it do something that breaks their understanding of what it is.

For my games, I have allowed PF to portray anything the spellcaster can think of, including fireballs and arrows, and whatever. But I also cap the damage that is possible due to this kind of illusion. If the caster creates a large ancient red dragon illusion breathing fire onto the target, I don't think its fair for that illusion to deal 88 points of damage, even on a failed save. That makes the spell a "save or die" kind of spell, akin to Power Word Kill, or Finger of Death. What I like to do is limit the damage to 2 points per level of the caster, so like Magic Missile, it gets more convincing and can do more real damage as the Illusionist gains levels. The damage potential should be lower than something like Magic Missile though, since the spell is far more adaptable to other uses than just causing damage. The best uses I see for pure illusions are not to cause damage, but to induce some sort of reaction from your foe based on what they are seeing.