You've got an itch to play a giant psionic slime-slug? Well, I'm not judging.
Aboleths have 8 RHD of the useless aberration type (worthless skills, too), but they also have a bunch of spell-like abilities (mostly illusion-themed). The spell-likes are decent, but probably won't be applicable in combat.
Aboleths also have three interesting natural abilities: living beings they touch have their skin transformed into a slimy membrane that must continuously be moistened and anyone getting close to them underwater may lose the ability to breathe air for several hours. Then again, anyone fighting an aboleth underwater is probably able to breathe water in some way. As a result, I don't consider those abilities valuable enough to actually affect LA.
The third, and by far most useful of those abilities, allows you to enslave any living creature for days. That's almost as good as Dominate Monster 3/day, except the enslaved creature gets a new saving throw every day. Minionmancy yay!
Aboleths also get some pretty fun feats in LoM, and their spell-likes can be made quite potent with the right support. In the end, I feel comfortable assigning a +1 level adjustment to this monster, considering how badly it suffers from its HD.
Finally, please enjoy this fun aboleth fact: all but one of their psionic abilities are from the bard list and aboleths have more charisma than intelligence. Therefore, an aboleth bard makes far more sense than the sample aboleth wizard they give us.