Target creature's Intelligence and Charisma scores each drop to 1. The affected creature is unable to use Intelligence- or Charisma-based skills, cast spells, understand language, or communicate coherently. Still, it knows who its friends are and can follow them and even protect them. The subject remains in this state until a heal, limited wish, miracle, or wish spell is used to cancel the effect of the feeblemind. A creature that can cast arcane spells, such as a sorcerer or a wizard, takes a –4 penalty on its saving throw.
By my understanding, the bolded section means that any spellcaster hit by feeblemind is unable to cast spells, not just one who uses INT or CHA to cast. Is this correct? I believe I am backed up by part of the spell specifically calling out arcane casters as having a worse effect happen to them than normal - leaving me with the thought that divine casters (including our WIS-based Clerics, Druids, and Inquisitors) cannot cast during the feeblemind either, although they are not subject to the -4 save penalty.
You are correct.
Feeblemind screws over any caster, but by not dumping their Wis it doesn't totally mess up their Will saves, which is important.
Pretty much any character/NPC is screwed when the intelligence drops to 1. One of my groups has taken advantage of this by following up with magic jar and then use the dumbfounded body as a shell.
Ah, that's brilliant! Thanks, folks - my BBEG's job just got that much easier. This fight is starting to look a little too hard after all - but hey, they deserve a nice terribly-difficult fight now and then, so it might as well be the last fight of the campaign!
Yes, Feeblemind in general was designed by divine casters specifically to use against arcanists.
QFT. The fact that it's not a divine spell, thereby leaving divine casters blameless for the devastation that ensues makes it all the more insidious.