I'm running BG2:EE on OSX, and now that I'm getting into the more Lich-heavy territory of the story, I've come to notice that Spell Immunity literally does nothing in my game.
When the spell is cast, I get no dialogue asking me to pick a school to protect against, and no effects are being applied to the character casting the spell. This effectively makes some fights near impossible since the first or second strike of some magic-using enemies is to cast Imprisonment on the main character.
No save + instakill + no protection = insta-game over, and that's just no fun. The only other option is to go for a Benny Hill-style chase and hope to be able to stay out of touch range, but since these enemies also have a nasty tendency to appear in tight spaces, that's not really a practical solution either.