Good, but from a DM point of view a total nightmare? (System within a system). Never played 3.5 but might investigate!

Oh, yes. A whole set of stuff bolted onto an existing ruleset that gives the psionic players extra spotlight time. 3.5 codified it more (psionics act like spells in 5e, attacks and defenses work like spells) but that took some of the fun out... made them just another spellcaster.