I'll be honest - I've never really loved psionics in any edition of D&D. In 1e, they were a confusing add-on, and its charts took up a lot of valuable DMG and DM Screen space. In 2e, they were pretty intensely broken. In 3e, they didn't work because of the crazy attribute requirements. In 3.5, they worked actually pretty well, but I didn't have too much interest.
In 4e, I still think the psionics system is flawed, but I'm slowly warming up to it. My concerns are mostly with the ability to spam low-level powers over and over again at high levels... Ardents and Psions both have powers at 1st level which are so good, I don't know why they'd ever trade them out, much less pay triple the points to enhance a higher-level power. Both the Ardent and the Battlemind seemed like space-filler classes - just psionic versions of Warlords and Fighters. The only one I really loved right from the outset is the monk.
Well, that's changed, after seeing some in play. I still have concerns re: those troublesome 1st-level powers; I still don't fully understand the logic behind upgrading your At-Wills; and I'm still thinking the power point system could have been much, much better. However, one of my players completely sold me on Ardents, and another impressed me with a Battlemind, so I'm warming to all of them. I see them as their own classes now, rather than psionic versions of the stuff we already have.
At any rate, this is quite a good splatbook. Like Primal Power, it has a large amount of flavor text; it's not all just powers, feats, and paragon paths. It gives you a better idea of what Ardents, Battleminds, Monks, and Psions do in the world - something pretty well missing from PHB3. Battleminds, despite all my expectations, are becoming one of my favorite classes.
There are some real gems here. Did you want your psion to light stuff on fire? We got you covered. Did you want your Battlemind to be able to duplicate himself and be two places at once? We got that, too. Did you want your Psion to have an at-will that teleports things? It's here! Did you want a Monk build that gives you perks for using weapons? Iron Soul has what you want. Did you want your Ardent to have a reason to trade out Disheartening Strike and Energizing Strike? Well... maybe not, but it sure tries.
I picked this up only because I'll be running Dark Sun soon, but now that I've read much of it, I'm glad I did. I recommend this to anyone who's not quite sold on the psionic classes yet, and to anyone who's running one now.