In Necrocarcerus, I use Courtney Campbell's Psionics supplement, which is an adaptation, rationalisation and upgrade of the AD&D 1e DMG Psionics system. It is an excellent free supplement.
After I ran a psionic combat the other day, one of the non-psionic PCs mentioned that it felt like in Shadowrun, when the decker starts hacking and everyone else gets up to use the bathroom, get a coffee, etc. until it's done. I feel like this comment is unfortunately accurate. While it was fun for the PC doing it, everyone else was confused and bored. I therefore find that I'm dissatisfied with with this system for several reasons, most of which derive from the source material (the AD&D 1e DMG's psionics system):
1) Psionic combat plays like an overly-complex mini-game that only involves psionic creatures. Psionic combat takes too long, requires too many calculations, and has too moving parts to understand their interaction in a straightforward way.
2) Psionic powers use points, but the value of any given point seems unclear, and many of your powers have different effects based on your total point value.
3) Psionics is too sealed off from other powers and non-psionic characters. Magic and psionics don't interact except in a handful of special cases.
4) Psionic combat and psionic powers seem really distinct from one another, like there are two power systems within the same class, which are used in different situations without much overlap.
On the other hand, there are several things about psionics as it currently stands that I like:
1) Psionics feels totally different than arcane or divine spellcasting, both stylistically and mechanically. I like that the resource allocation decisions differ between its pool-based system, and the slot-based system of spellcasting.
2) Psionic combat is a cool concept. I like the idea of different attack modes having different effects.
3) I like the distinction between minor powers that are cheap and easy to use, and major powers that are not, but are broader in their application. More generally, the powers outside of psionic combat are mostly interesting, flavourful, and well-designed.
All of this is a prelude to a larger rewrite of the psionics system. While it isn't fully worked out by any means (I'm concentrating on the Necrocarcerus rules document and aiming to finish v.1.2 ASAP) here are some ideas I'm kicking around. Some of these are adaptations from both Courtney's spell pool system from Numenhalla and AD&D 2e's Complete Psionics Handbook.
1) Magic and psionics are interchangeable. Anti-magic shells block psionics, psionic barriers block magical mental effects, my shield blocks your fire darts and vice versa.
2) Psionicists characters and creatures have a power pool equal to their HD. Wild talents have a power pool equal to half their hit dice. You recharge your power pool by resting overnight.
i.e. an 8th level Psioncist in Necrocarcerus has 8d8 to manifest powers, fight in psionic combat, etc. with (because Psionicists use d8 HD in Necrocarcerus). A 10th level wizard with a wild talent rolls 5d4.The Lesser Dreams of the Ghoul Star (a sort of embodied undead fragment of an intelligent, malign star) roll 7d12 because they have 7 HD, and those HD are d12s.
Wizards are crappy at psionics because their brains are mostly full of spells.
3) Instead of points spent, powers require the psionicist to roll dice from the power pool and exceed a target number. To maintain powers once activated, dice from the power pool must be dedicated to the power. The number of dice rolled to manifest the power is the the "Mastery" level the power manifests at, and the dice dedicated depend on the level of the power - 2 for a discipline, 4 for a science, 8 for a grand art. Dice that come up less then 3+ when manifesting a discipline, 5+ when manifesting a science and 9+ when manifesting a grand art are lost from the power pool until the character rests (i.e. you always burn the dice you lose manifesting a grand art).
(The hard/boring part of actually making this system will be restatting out all the powers)
4) Psionic combat uses grapple rules. Courtney and I both use a system where grappling is handled by both sides rolling their hit dice with the winner pinning or stunning the other side.
In this, psionicists and psionically active creatures can initiate psionic combat against any intelligent creature they can see (provided they aren't already in psionic combat) without needing an attack roll. Psionicists and psionically-active creatures roll their power pool. Non-psions have a penalty, either of the number of dice (possibly half their level or HD) or the type (d4s vs. the usual d8 of a psionic character or creature).
If the defender wins, the attacker is stunned for a round and loses a die from their power pool until they rest. If the attacker wins, then the defender loses all dice from their power pool and they render the defender helpless until they choose to retreat from their mind.
5) The different attack and defense modes would each change this dynamic in one way or another. Attackers and defenders choose their modes of attack and defense respectively when they decide to initiate psionic combat.
Psionic Wave would let you grapple multiple opponents. Psychic Crush would let you cause real damage to your opponent. Id Insinuation would let you charm or frighten an opponent instead of rendering them helpless. Ego Lash would let you puppet them (while your own body remains helpless) and Mind Knife would upgrade your die type by two but wouldn't cause any other effect upon success. The attacker selects when they first initiate psionic combat which they're using.
Empty Mind causes the attempt to initiate psionic combat to fail unless the attacker first passes a saving throw. Shield Thoughts allows you to defend even when you are surprised, unaware, dazed, stunned, unconscious, asleep, etc. Fortress of Intellect lets you both interfere in psionic combat between two other characters and drag your nearby nonpsionic friends & drones into the combat to add their dice pools to your own. Spire of Iron Will lets you increase your die type by one. Cerebral Barrier nixes any other mental effects you're under, and causes you to merely be stunned for one round if you lose the psionic combat.
These are just preliminary notes, obviously, not a fully worked out system at this point.