Posting should get more regular from now on: here's an extra-long post to make up for the recent lack of a schedule.
Mind flayers are medium aberrations with 8 RHD. They possess high ability scores all around, especially in the mental department, but even so are not very impressive chassis-wise.
Fortunately, there's a wonderful variety of special abilities available to them. Their SR (25 + class levels) is insanely high, their 100 ft. telepathy is awesome combined with Mindsight, and their SLAs are all useful to various degrees, even at higher levels.
Then there's Mind Blast. Mind Blast creates a huge cone of save-or-be-stunned, has a respectable duration, can be used at-will, and is arguably not even mind-affecting. If you have strong-willed, stun-immune or otherwise blast-resistant allies, it's even better.
Illithids also have Improved Grab and Extract, but the first shouldn't be bothered with. At medium size with only a small strength bonus to amplify your medium BAB you're not grappling anything soon: and why would you with such a beautiful engine of incapacitation at your command? Extract is interesting, but by the time you've managed to quadruple-grapple something you probably could've stunned in three times over.
Finally, there's a small amount of illithid support outside of core. Graft Illithid Flesh gives access to a number of interesting items (try outfitting your rogue with a few nice natural weapons), albeit at the cost of damaging the will of whoever uses them.
Illithid Body Tamer is an attempt to turn illithid into a melee character, which is... less than advisable.
And then there's Illithid Savant: a strong contender for the most powerful PrC in the game. It's similar to the beholder mage in being an unbalanced option for a single race consider inaccessible for PCs. However, where beholder mage has some kind of natural upper limit only TO will break (even if it's: 'cast ten spells as a free action per turn), the only end point for illithid savants is to basically become Pun-Pun.
As one might expect, balancing this is hard. Even at the current LA of +7, a mind flayer entering Illithid Savant will have obtained all important class features by ECL 20, at which point there's literally no limit to its power.
Even worse, Illithid Savant is basically the only feasible way to advance a mind flayer: its only other race-specific PrC is crap and it doesn't have any casting to improve.
Then again, I can hardly raise their LA to +12 just to prevent illithid savant cheese. 9th-level spells are OP, but nobody is going to argue half-elves deserve +4 LA because they'd otherwise able to obtain them.
Instead, I'm going for a LA of +2* (asterisk for at-will astral projection). Non-savants should be reasonably balanced this way, while savants aren't getting their major power boosts (stealing class features and special abilities) until casters have respectively 7th- and 8th-level spells.
Feedback is anticipated.
Next are minotaurs!