Let's recap: the past four monsters have been a bluish goblin, a psychic rodent, a hideous tortured spectre, and a demon with a severe case of the ol' spinebrains.
Cerebriliths have nine outsider RHD, which allows me to be carefully optimistic about their playability. They're large-sized, with the good stats expected of a mid-level demon. 100 ft. telepathy is good, DR 10 is good, energy resistances/immunities are good, not much to complain about there. +8 to listen and spot are a bit weird, but not unwelcome.
Natural weapons are very underwhelming, especially for a 9 HD Large critter, with a 1d8/1d6 bite/claw/claw routine. Ah well, not like this needs to rely on natural attacks a lot.
Its summoning ability lets it call a horde (4d6) of dretches, or another cerebrilith, both with a 35% success rate. The low percentage there stings, but even so it's significant enough to shake up one encounter every few days.
Interestingly, the cerebrilith has psi-like and spell-like abilities.
At-will psi-likes are Brain Lock that can be used on any nonmindless foe (surprisingly good, especially on a PC), 2d4 Ego Whip (a solid backup option), Detect Psionics, four-target Id Insuniation, and Mind Trap. Those are hardly terrible abilities, and definitely relevant at the levels cerebriliths become playable.
Further psi-likes are 3/day Psionic Dominate, Ectoplasmic Form, and Mind Probe, as well as 1/day 9d10 Mind Thrust. The last of those is pretty underwhelming, but the other three provide a healthy mix of utility and combat strength.
The spell-likes are at-will Deeper Darkness, Desecrate, Detect Good, Greater Teleport (self + 50 lb., as usual), Magic Circle Against Good, and Unholy Blight. Some of it is pretty underwhelming, but most of those definitely have their uses.
As far as advancement goes, the cerebrilith has access to the various 'Fiend of' classes, as well as a number of other PrCs. This leads me to believe that even without an obvious class to take, it'll be able to stay relevant at higher levels.
Considering the cerebrilith's ability to near-indefinitely daze unprotected targets, amazing range of utility magic, and fair amount of combat abilities, in addition to a strong chassis, and looking back to how demons have been traditionally rated, I feel like +1 fits fine here.