Here's a post I just wrote about "Feeblemind vs Disintegrate" for the ToB Forum (If you want to know my Opinion on this great level 5 spell)
For the people who never really used this spell : Try it out!!!If you play a Sorcerer, just use a Greater Malison followed by a few Feeblemind. Then you'll have a great training Dummy.
Feeblemind
Saving Throw vs spells (at -2)
Give you a dumb enemy to bash
Spell lvl 5
Disintegrate
Saving Throw vs spells (No penality)
Destroy an opponement (and the loot)
Spell lvl 6
I just... HATE disintegrate... Why do people use it? Feeblemind is just WAY better! Sure you have to kill the enemy after, but that's just a matter of 1 minute.
S P O I L E R
Listen to my story (kids ) : I was just hunting down that Black Dragon guarding the cup in the Elven city. I was killed every time (I was pretty weak). One time, I just memorized loads of Lower Magic Resistance (With Aerie) and a few Feeblemind spells with Edwin. Aerie cast several Lower Magic Resistance while Edwin was invoking some minions and hasting my warriors. After that, I had one choice : Cast a Feeblemind and cross my fingers, or just lauch loads of Magic Missibles, Melfs Acide Arrows and Fire Arrows. My feeblemind just transformed the Dragon in a Giant Flesh Statue. That's how I discovered that magnific spell : Feeblemind.
BTW : It works really great on "small" monsters like Umber Hulks, Lesser Deamons, Golems (But they have a great Magic Resist), etc.