It's one of those books that's actually difficult to read; mostly mechanics with very little flavor. That said, psionics has always interested me as a totally new way of doing magic. I don't actually think that psionics alongside divine and arcane magic is all that interesting, but psionics instead of divine and arcane magic is a topic that fascinates me.
In any case, the book is pretty solid. The mechanics are cleaned up and better all 'round than the original 3e Psionics Handbook. I've gotta believe that this product only serves a niche market, but I happen to fall into that niche.
The notable thing to me is that psionics is very much simply magic. Lots of D&D players will tell you (or at least I've seen this expressed frequently) that psionics is too "science fictiony" for their tastes. This obviously doesn't turn me off, because I've expressed here before how I like the combination of science fiction and fantasy (and horror) elements in the old-school, classic "weird tale", but in this case, it's particularly irrelevent. There really isn't anything science fictiony about psionics, at least as presented here. It's really just nothing more than magic, except with different mechanics.