Lets see, I'll compare a Cleric/Drider Cultist to a core drider and see what's different ability-wise. Using cleric since that has the same size of HD as aberration (sor/wiz/psion driders will have less hp than core)
HD: 7d8 compared to 6d8, the cleric is a bit better off
BAB and base saves(F/R/W): +5, +4, +2, +7 compared to +4, +2, +2, +5. The two saves are better because of the way multiclassing works and extra HD, BAB is better due to the extra HD.
Spellcasting: 5th level cleric compared to 6th level cleric, core drider is better off here.
CR: both are CR 7
ECL: C/DC is 7 compared to the 10 of the Drider
Ability scores: C/DC loses +2 from Dex and Con (if human) or +4 from Con (if elf)
Special Abilities: C/DC loses SR, a host of spell-like abilities and racial bonuses to Hide and Move Silently skills.
All in all, I'd say it's a good class, though I wonder if 4 levels is too good for such adjustments to ability scores. I might move the spellcasting advancements to even levels and add a fifth level, but then I can't decide which abilities, if any, would be moved to the new level besides transformation.