The Codex is more than just a translation of old D&D spellcasting tropes into a sci-fi context. Crawford's apparent design philosophy for spellcasters (that they should be doing the impossible, rather than just outshining the Warrior and the Expert when they spend spell slots) results in a library of new classes, spells, and foci that feel magical without falling into the "quadratic wizard" trap. The most potent spells have costs and limitations that demand more careful planning and risk management from spellcasters who want to get maximum value from their spell slots.
The sourcebook is flavorful too, providing new mysterious creatures called Shadows that can be enemies, summoned servants, or figures of worship that grant cultists twisted powers. Classes are accompanied by some default backstory and flavor to help players and GMs explain the otherworldly origins of their powers, and understand how magic-users might be judged by society. And for those GMs comfortable with a bigger bookkeeping burden, arcane research rules offer paths to power with built-in reasons to adventure for the research necessary to summon permenant minions, expand Arcanist spellbooks, and craft magical items.
If you don't want magic users in your sci-fi, obviously, pass on this one. If you do want them, buy the Codex without hesitation.
[5 of 5 Stars!]