Psionics offers IMO a wider array of effects that go beyond the standards in the Pre-Spell Compendium Arcane and Divine magical abilities. Before the spell compendium there was nothing like Dimensional Twister or Etheral Abduction, which are both killer effects. The loss of a truly effective TK however is really annoying in 3.5 Psionics.Share your thoughts, please!
Psis also work better together than other casters thanks to powers like Metaconcert. This makes basing a campaign around Psions a more cohesive option than other "one trick pony" campaign ideas.
Finally, the augmentations make psis more flexible than similar spontaneous arcane & divine casters (sorcerer and favored soul).
One weakness is I always wish they would've made *some* rules for creating additional augmentations. Looking at D-Door in Complete Psi shows how many options and how varied the options can be for a single power.
This of course varies depending on what you view as "transparency" as well as the old faithful "rules as written vs. spirit of the rules" issue. Tranparency makes for weirdness and that's all there is to it. For example IMO the Psychic Vampire power should be able to suck spell slots in a transparent game.Well, no. The rods allow you to use a metamagic feat for free. Despite the transparency, psions use different feat for their meta-stuff. You'd bump into an increase in "spell level slots" that simply doesn't apply.