I went through the first intellect devourer fight and as a DM I think this fight is a bit out of line. The player has two characters who will have around 8-10 HP on the average and the IDs will do enough to kill one of them in two blows. Ranged fire is unreliable, I generally do no more than 3 points of damage with a bow. I beat the fight by running a cleric alongside Blackheart with both using shield of faith and guiding bolt, options that I think surpass what most characters have available at that point in the game.
I agree, though my first Intellect Devourer problems began after the ship crashed. There were 3 or 4 of them. They were tearing my party to pieces.
Perhaps there should be an "Easy" mode, where you gain +5 on every die roll. Or "Very Easy" +10! I would like to be able to do a playthrough without saving.