Advice from the trenches, as it were
Three basic points to remember about a psi-centered game.
1. They don't have any good healing. Compared to a cleric, it's just about nonexistant.
2. Their combat stats are lower than a normal group, both in attack and hit points. They tend to have solid ACs, but are weak on the offensive and are more fragile than normal PCs, due to lower Hit Die and reliance on more stats.
3. They need prep time. If they don't have a round or two to activate combat-related powers, they aren't even going to be close to a normal group. A PsyWar is nasty when he's got some powers going, but if he doesn't, he's just a cleric with a poor Will save. And he's going to get eaten alive by a comparable fighter.
I have actually played in an all-Psi game, though it was before the release of ITCK and Of Sound Mind. Everyone was required to have at least half their levels in psionic-using classes (Psion, Psychic Warrior, or Psychic Rogue). We were playing githzerai on the race's home world, trying to regain control. Essentially, we were told to go out into a section of jungle and clear it of dangerous creatures and find out what we could about what was out there. The DM did a good job of building an interesting background for the setting, and it worked out well - for a while.
The PCs were, as well as I can recall them:
En'kun - Mnk1/PsyWar5 (me, used Claws of the Bear and Talons, along with the Spring Attack tree, to do quite a bit of damage with my unarmed attacks - usually on the order of 1d12+7 @ +9/+9)
PsyWar6 (spiked chain master - quite nasty stuff, with Expertise, Improved Trip, WF/WS, etc)
Psion6 (Telepath) (no real focus; her best bit was Inflict Pain with Trigger Power, which she could get off pretty reliably)
Psion5 (Savant)/Pyrokineticist1 (fire guy, though he wasn't very good at it)
Psion6 (Shaper) (summoned hordes and hordes of little monsters - quite nasty, really, as he could summon a whole lot of the little buggers)
PsyRogue6 (a fairly standard rogue-type; his powers were mostly wasted, the only one I remember him using was Invisibility)
PsyWar1/Psion (Metabolicist)4/PsiHealer1 (the healer of the group - she had some custom healing powers and a level in a custom Psionic Healer PrC. Quite high AC, as I recall - she took her level in PsyWar to get plate mail and a halberd.)
PsyWar5/Zerth Knight1 (paladin-esque character, the Zerth Knight gets a few odd powers, but is mostly a psionic paladin)
PsyWar5/Slayer1 (the token illithid hunter, necessary for any group of githzerai. Since we never met any illithids, he didn't do too well, though he was a decent - not great - warrior in his own right).
The problems started with a pack of trolls. There were about eight of them, as I recall, against nine 6th level PCs. They surprised us, and in the jungle, encounters start very, very close. I believe they started within 20' of us, much less than their charge distance. As we were all caught by surprise, we had almost no powers up. I believe I (a Mnk1/PsyWar5) had Claws of the Bear up, as I almost always did. And that was about it. It was a horrible fight for us - they ripped us apart completely. We didn't manage to take a single one of them out, and had to retreat quite rapidly. I think only one of us actually died, but I know that it was down to three of us standing at one point. That would have been me, the healer, and the spiked chain master. While mechanically, we should have been able to deal with them without much of a problem, we just got our tails handed to us in that fight. No one had the hit points to deal with them, and we had no real way of damaging several of them at once. Our psionic attacks were useless, as they had the nonpsionic buffer. Our attacks weren't strong enough to bring them down consistently.
So, we ran from that fight. After some exploring, we did return and ambush the trolls ourselves. That time, we decimated them - not a single PC even dropped, and all the trolls were wiped out. It all came down to prep time. A psi party NEEDS to have time to activate its psionic powers, or it will get creamed by a group of monsters who don't rely on magic.
After we did some other stuff (fighting degenerate gith, su-monsters, and a canopy dragon, not to mention meeting with another tribe of proto-gith), we went back out. This time we encountered a group of githyanki, and they also decimated us. There were several knights (6th level PsyWars, as I recall - SavageWombat, you remember?), and two Psions. They were essentially a group like us, except they had good teamwork. They'd pick a single target, and all the knights would hammer them until they fell. It was brutal. We just had no way of dealing with them - we couldn't hit them and they hit us just about every time.
In the end, the group decided that we'd shelve that game and do something else. I regretted it, since I loved my character. The DM gave us all the opportunity to rewrite our characters now that we'd played with the psi stuff, but most of the group didn't want to. Therefore, we moved on to a more conventional fantasy game. The DM and I had a fairly ongoing conversation about the relative strength of a normal party vs a theme party, and I still maintain what I first said. A party that is focused around one particular class - no matter which one - will be weaker than a comparable balanced group in the long run. Just something to keep in mind from a guy who's been in a psionic campaign.