Over 25 years of gaming, I have run all of four campaigns that I consider to be truly excellent. That may not sound like much, but it's worth noting that I've run many 'good' campaigns, and can consistently run an entertaining campaign; it's just that these four stand apart. But of course, none of the four are perfect; each has taught me various lessons about gaming. The first, if I'm honest, was a "Lord of the Rings" rip-off based on a far from perfect comprehension of the rules. "Vampire: Rivers of Time" was excellent for a long time, but it ran on far too long, and ran out of steam towards the end. "Shackled City" highlighted a lot of the systematic issues with 3e, especially at higher levels.
Which brings us to "The Eberron Code", my current and ongoing campaign. Which has a number of lessons to share. However, with the campaign still being ongoing, I don't really want to start analysing most of these. One, though, has ceased to be an ongoing component in the campaign, and so can be discussed here: psionics.
This is the first 3.5e campaign I have run to include psionics to any significant degree, with the inclusion of a psionic PC, a number of psionic items, and a few psionic monsters. With the death of that psionic PC, the role of this power source in the campaign is essentially ended.
Frankly, and rather unfortunately, psionics are a waste of time and effort.
It's not a balance issue - if anything, psionics are underpowered compared to magic. There are just a few powers that are overpowered, but compared with some of the abuses possible with a Wizard, Cleric, or Druid (especially with the "Spell Compendium" in play), they're barely worth mentioning.
The problem is that psionics is a bolt-on system to a game with a fairly comprehensive magic system. For just about any psionic power, there's already a spell that does it available to one of the three classes I named.
Additionally, when designing these spells, WotC worked out the lowest level at which the power should reasonably become available, and used that to assign the spell level. Sure, not all casters get all spells at that lowest level - the three types of magic give different effects at different rates. But one of the 'big three' will get every spell at the lowest applicable level.
The effect of these two things is that the psion has virtually no unique powers reserved to himself, and he can't even give earlier access to any existing spell (because one of the 'big three' already gives it at the lowest acceptable level). All the psion can really offer, in terms of the powers available, is a particular mix of powers at their lowest levels. Which would be fine if there were huge disparities in spells available to the 'big three', but there aren't - most often, spells are granted at the same level, or very close to it.
The one thing that the psion does offer is a different spellcasting mechanic. Where the Wizard prepares his spells, and the Sorcerer spontaneously casts a wider number of spells but still uses slots of different levels, the Psion has a pool of points that can be used to power any effect at any time. In effect, the Psion is a mana-based caster, albeit with overnight recharge rather than the more common "points per hour" seen in CRPGs.
That being the case, I'm inclined not to use psionics as such in future campaigns. They just add a bunch of new complexity without really offering anything. Instead, I might be tempted to do three things:
1) Introduce the Mage, a mana-based arcane caster to stand alongside the Wizard and Sorcerer (and perhaps a divine equivalent to stand with the Cleric and Favoured Soul, and perhaps even a primal equivalent to match the Druid). This class would be modelled on the rules of the Psion, but would have access to the existing spell list for the 'parent' class. If nothing else, this gives that character access to the full range of spells in all the expanded supplements, if used.
2) Model 'psionics' using the "spell template" rules from Arcana Evolved - which allows the existing spells to be reskinned as psionic effects, without a need to introduce lots of new powers or huge numbers of new rules to handle them. That's if psionics are to feature in the campaign at all (for Eberron or Dark Sun, I would certainly want to use them; for other campaigns I don't think I'd bother).
3) The vast majority of psionic items can either be easily converted to magic items (as is actually done in the "Magic Item Compendium"). The vast majority of the others can simply be dropped. The one that I think is useful to retain is the Cognizance Crystal, which gives a psion additional power points. This should be easy to convert - when charged, it gives a source of additional Mana. However, a Wizard can also use it to cast a single prepared spell of the given level without expending the spell, or a Sorcerer can use it as an additional slot of the given level. Which, coincidentally, makes this just an additional use of the pearl of power.
And that, I think, is that. I think the next "Lesson" may discuss the use of the "Spell Compendium", but that's going to be a while away...