What freaks me out about Feeblemind can be reduced down to one big thing and two small things.
The big thing is what it takes to undo the spell; there is nothing that can be done for a Feebleminded character beyond Heal, Limited Wish, Wish, or Miracle. All of which, I will note, are higher-level spells than Feeblemind itself. For only a fifth level spell, that's almost unbelievably harsh; in many campaigns, it could remove a character more effectively than simply killing them. Hell, if I had a choice between being hit with Feeblemind or dying, I'd take death: at least the cleric only has to be able to cast fifth-level spells (Raise Dead) to fix being dead.
The small things that worry me are the penalty to the Will save for arcane casters, which is really just adding insult to injury, and the absence of a duration for the spell. If it lasted a week or a day or even an hour per caster level, it would still be a kick-ass spell...but no, it lasts until the victim dies or someone manages to find a cleric who is willing and able to cast Heal. That's just evil.
My wizard character in our current game has the spell and uses it, of course. I feel bad about it, but it's just too devastating a spell to not use. But if I were running the game, I'd be house-ruling Feeblemind in some way, either giving it a limited duration or saying that it can be removed by something less potent than Heal. And if I couldn't pick a suitable 5th-level spell, I'd just invent one to specifically cure Feeblemind and stick it on the cleric's list. Because good lord, I don't think it should be easier to bring someone back to life than it is to undo a lobotomy enchantment.
I think Feeblemind would still be a useful spell even with that kind of modification, and it'd be less likely to completely nuke a character right out of the game.
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right now, it tends to encourage mutually-assured destruction showdowns
ryan