My group is a big fan of it; it's got a pretty central role in races, if not classes, IMC.
I'll go through the list:
Dromites: No one has played one yet, but they've met them as an industrious low-caste crafting race (they work with the base materials that the elites are a cut above).
Duergar: About to meet them as a BIG force of unpleasantness in their current incarnation, as some obnoxious people protecting a great evil god (mostly because the were tainted by it).
Elans: The elite upper caste IMC, these mystics are the kings and queens of the land...but surprisingly rare...
Githyanki: With the amount of planar holes IMC, the Githyanki are known as a militaristic people from beyond, often recruited as warrior-caste.
Githzerai: Well, these guys are *responsible* for the holes in space-time, so...yeah, they're important.
Half-Giants: Got a guy playing a Psychic Warrior Half-Giant who specializes in claws and bites; he just died to the Duergar's little helpers. Half-giants are re-named Giantkin IMC (there's no half-races), and they're a slave race of laborers (guy's now going to be playing a Ceptu Nomad...heh.)
Maenads: An elite race of priests 'siezed by the gods' when they Outburst, these guys are about to become a big political problem.
Thri-Kreen: Desert-folk on the outskirts, recently almost obliterated by the Dark God Ophiuchus. They've been a constant neutral for the PC's.
Xephs: One of the sorority sistas played a Xeph swashbuckler and was very happy with it, but it died in the implementation. These are foreigners, again, from the coast.
Psions: My own house rule keeps in the varied ability scores determined by discipline, except that one ability score governs all the powers...Egoist is Con, Kineticist is Int, Nomad is Dex, Seer is Wis, Shaper is Str, and Telepath is Cha (this combines nicely with my already-implemented
Polymorph house rules). This has worked out swimmingly so far. These are the religious figures IMC.
Psychic Warrior: The guy who played a half-giant was one of these specialized in claws and bites, and was a bit of a machine, designed to provoke agro mostly....I had to use some mind-affecting things to screw 'em over this time.

These guys are the primary military warrior caste.
Soulknife: Meh. It's great balance-wise, but it's a bit flavorless; they're a special monk-like order IMC, specifically for those warriors who don't belong to a caste already (since being a Soulknife makes you an Untouchable).
Wilder: Great for wacky forest hermits, the Wilder hasn't seen a lot of use yet...
Powers: So far, so good.
Prestige Classes: The Ceptu is going for Metamind....*shrug*.
Monsters: I have had a Blue PC (who was a Conjurer), and he seemed balanced, though he didn't take any pshychic powers. I also may be having an Unbodied PC on my hands soon, but we'll see how she likes not having levels...