Hmm, let's look at it piece by piece.
jRocket said:
Its a tiny undead, it can fly, and has blindsight and telepathy.
Bam! Good start! I give this +3 LA right here- according to Savage Species, flight with good maneuverability is worth +2, and blindsight is worth another +1. Undead immunities, imho, is almost worth another +1 but we'll pass that one for now.
jRocket said:
+6 int and cha, +4 dex, +2 wis
Another +1 LA. We're up to +4 at this point.
jRocket said:
Naturally, since it has no real body, it has no attacks whatsoever and has no str or con. The only way it can manipulate objects is thought telekenesis. Thats a major drawback there that should be taken into account.
It has a "mind thrust" which does 2d10 damage to anyone that fails a will save.
This is roughly the equivalent to a weapon attack for a BiaJ. And it's a pretty darn big weapon. Yes, a target might make its save; this is analogous to missing on your attack roll. I give this another +1. We're at +5.
jRocket said:
It's "Madness" ability drains Wisdom from anyone that tries to use thought detection or mind control against it.
It has a suggestion and telekenesis 3x/day and dominate person 1x/day
It can also rebuke or command undead as a cleric.
Another +1 here for spell-like abilities, total +6.
Now, let's compare a brain in a jar wizard 1 (total ECL 10) with a 10th-level human wizard.
HP: the wizard averages 25; the BiaJ averages 22. Pretty much a wash here- minor advantage to the wizard.
Spells: the wizard has a lot more spells. The BiaJ's best abilities are telekinesis and dominate person- 5th level tops, iirc. But it has less of 'em- advantage wizard. However, all things being equal, the BiaJ tops the wizard in save DCs (+6 int!).
BAB: wizard +5, BiaJ irrelevent. The only thing that matters is your Will save. Hmm. I'd say this is a wash- but the BiaJ does more damage with its 'standard attack mode' than the wizard with a weapon. Of course the wizard's got tons of low-level spells, too...
Saves: Wizard has +7 Will, +3 Fort and Ref (base). BiaJ has +5 Will, +1 Fort and Ref for base saves. Advantage to the wizard.
Seems like +6 is too high, but not by much. I'd go with +5.