The Intellect Devourer is a solid -0*.

It is massively dependent upon Body Thief to have a chance of regularly contributing, far less taking an regular and active role as a character.
Without Body Thief it could maybe slide as a minion (ie, Improved Familiar-esque), where the ability to communicate rests with the main character, not the minion.'


And Body Thief is a massive DM caution. Plus it's a pain in the ass as a player - you can only use each body once, and only for a maximum of 7 days. This isn't the kind of logistical issue that can be handwaved away (ie, a lot of DMs won't bother to track individual mundane arrows (or other low-value mundane consumables), but you can be sure as that the DM is going to keep a close eye on your body supply ... and the trail of corpses.