According to 2nd edition AD&D, which the BG series is based off of, the Imprisonment spell should behave quite differently than it does in the game. For one, it's a touch attack; that is, you must actually reach out and touch the victim in order for the spell to take effect. In BG2 and ToB, you need only cast it, and voila, they're history. That makes this spell ridiculously powerful. In fact, once you start casting it, the result is final; even if the victim runs completely off the screen, your Imprisonment spell will actually follow them! Nutty.
I couldn't say whether or not this is true for BG2, but in ToB, magic resistance apparently has no bearing on Imprisonment whatsoever; which is bizarre, considering that it's a directly targeted spell. My monk has 103% magic resistance, and that demi-lich tagged him every time. My sorcerer could also imprison virtually anything, regardless of strong natural resistance to magic. This aspect I can assume is a bug, though it may have been intended to behave that way. I certainly hope not.
Anyway, I was frustrated, so I decided to b!tch to all of you about it. I feel much better now.