Intellect devourers: 6 HD small-sized psionic aberrations that take over your skull, eat your brain, and walk around in your corpse. Charming.
Ability-wise, the only stat that stands out is 21 dexterity. Everything else gets small (+2 to +4) bonuses. Defensive qualities are quite impressive: DR 10/adamantine, PR 23, fire immunity, and electricity resistance are all pretty good things to have. +5 natural armor is useful too (you try finding armor that fits a walking brain), but the four natural claws are garbage (1d3 damage does not a viable attack make).
Of course, it's the special abilities that are really interesting here. Chief amongst them is Body Thief, which allows the intellect devourer to take over the body of a helpless or dead creature as long as it's not undead, a construct, or wearing a +1 heavy fortification buckler. The size limit is kind of weird: everything Small and up is fair game. Any time someone asks you how your devourer is merging with a kobold smaller than itself, point at the (Su) denotation and shrug. The only small downside is that you can't control a specific body for more than seven days.
Taking over a body, in addition to the important cosmetic effects, allows you to learn the victim's former languages and gives you essentially everything a Polymorph spell would grant (with the obvious exception that you're not turning into a general creature, but one specific example). Interestingly, this takeover might very well include certain class features (up to and including spellcasting). Ask your DM, and wear a helmet when you do so.
Aside from this, intellect devourers also have blindsight (60 ft.) and some PLAs. Those are at-will Cloud Mind, Detect Psionics, 2d4 Ego Whip, +5 Empty Mind, and three-target Id Insinuation. They also get 3/day 2d12 Body Adjustment, Intellect Fortress, and Painful Strike. Of those, a few are just garbage (I'll just go use Painful Strike on my 1d3 claws, that sounds smart), but most are decent. The lack of scaling is a bit annoying, though.
Finally there's the strange weakness that Protection From Evil prevents intellect devourers (who haven't lost their evil subtype somehow) from touching the affected creature, as if the devourer were summoned.
Intellect devourers are tricky to rate. Early on, I suppose their good defenses and moderate-to-okay PLAs can let them stand their own in combat, especially if they get a good body thrown their way every few days. Later on, however, they suffer from a lack of obvious advancement. While Body Thief need never become obsolete, the other abilities become less and less useful as time goes on.
In the end, +1 is a fair rating. The devourer can use (abuse?) Body Thief to great lengths, but not to the extent that it needs an asterisk. Losing a level is a fair penalty, though.