It might sound like faint praise to say that this product is "as good as it can be all things considered", but in the context of designing a OSR psionics system, that's actually a compliment. Anyone attempting to integrate old-school psionics (one of the most notoriously fiddly bits of AD&D) into the streamlined minimalist B/X model of play is really attempting to square a circle. This product executes some parts of that project commendably well, and even the parts where it doesn't succeed still manages to be interesting and worthy of study. I've been waiting a long time for its release, and even planned certain elements of my campaign around its anticipated release.
The editing is high quality; I found a table that listed "min meight" instead of "min height" (weight?) on page 13, but no other issues. The illustrations have a retro minimalist feel and do a good job of invoking the theme of psychic warfare. There's high-quality index and a separate set of tables at the end that could be clipped and integrated into an existing GM screen.
To give potential purchasers some idea of what they're buying, this product ("the Basic Psionics Handbook", henceforth PX1) needs some short description. The major selling point of PX1 is that it reconstructs the original vision for a dedicated psionicist class (the "Mystic") based on communication with its creator, Steve Marsh. The first publication of a D&D psionics system (in the Eldritch Wizardry supplement) essentially decided to disassemble Marsh's class for its spare parts, reconstructing it into the "wild psionics" system that passed over into 1st Edition AD&D. The PX1 psionicist isn't a perfect reconstruction of that lost class, but it's as close as we're probably ever going to see. In particular, it also restores some of the original cultural flavor of the class abilities, which were clearly inspired by the various siddhi of Hindu mythology.
The basic mechanical modification to the old AD&D system is that all abilities (disciplines and attack modes) now require a saving throw vs paralysis. The number of psionic strength points has been scaled down for mathematical simplicity, and the various defense modes now provide modifiers to both the saving throw and to the damage/duration of the attack effects. This effectively reduces the huge combat matrix tables in 1st Edition down to a single 5x5 attack-defense table, and also lets the mystic play a little more like a standard caster class that piggybacks on existing attributes and saving throw statistics. The tedious requirement to track hourly regeneration is mercifully replaced by a full-restore of points by a single night-rest, as with other BX/LL casters.
But there's plenty of other useful stuff in here too. There's a reorganization and balancing of the various disciplines into thematic categories similar to later psionics systems, but with an eye toward keeping a short list that highlights some of the early "classic" powers. There's a convenient collection of some of the iconic 1st Edition psionic monsters, with slight modifications to fit with new mechanics and avoid copyright infringement. And for purists who want to retain the old "wild psionics" approach, there's a short appendix outlining a way to assign random talents to other classes.
If there's a weakness to PX1, it's in the implementation of attack and defense modes. On the one hand, it does preserve most of the cosmetic aspects of the old system, retaining all the names as well as the idea of cross-referencing them on a grid to determine the results of mental dueling. But the cross-pollination between 1st and 2nd edition ideas leaves the interactions feeling more muddled. Instead of just having damage modifications (1e) or having saving throw modifiers (2e), there's now a combination of both, a redundancy that suggests overly dissociated mechanics. In particular, this leaves the "high" defense modes (Intellect Fortress and Tower of Iron Will) seeming generic, with no particular strengths or weaknesses -- in contrast to the 1e grid where Id Insinuation can strongly counter those area-defense modes, making them dangerous to overuse.
Another oddity that probably reflects vestigial elements of abandoned mechanics is that Psychic Crush (now simplified to a direct damage ability) is still priced higher than all other attack modes. This is something that made more sense back in 1e, where it could cause instant death. But in PX1 it's hard to see any good reason to use Psychic Crush over Psionic Blast, an area attack that also inflicts a devastating multi-round stun despite being cheaper. It's like a magic system where Fireball is a 3rd level spell, but Magic Missile is 4th level.
But probably the biggest problem is that the system maintains a museum-curator's fidelity toward one of the peculiar aspects of the 1e system: the tendency of weak psionic ability to actually be a curse rather than a blessing. The developer acknowledges this after the presentation of the new Monk class, a martial artist with some limited psionic empowerment. Once this class's tiny pool of psionic strength points is depleted, it takes hit-point damage from every psionic attack. Yet even worse is the fate of wildcard psionics users, who could see their pools depleted by a single attack -- because they'll only gain enough PSP to cast their lone wildcard ability. This is actually more punitive than in 1st Edition, which at least gave wildcard users a deeper buff of attack/defense points based on their latent "psychic potential". (Well, assuming you did well on that percentile roll...) With a GM who has any tendency to throw psychic monsters at you (i.e., any GM using this product), you absolutely DON'T want to be psionically awakened as a wildcard!
It's worth comparing PX1 to the much simpler (two-page) psionics rules created for LL by Procter and Curtis in Realms of Crawling Chaos. The latter makes no pretense of having any purpose other than giving alien abilities to a few Lovecraftian monsters, but it actually does a better job of carrying forward some of the most fearsome thematic aspects of AD&D psionics. For example, it keeps Psychic Crush as the "psychic version of death spell", it allows Id Insinuation to retain its mind control function, it allows for the long-term coma effect from Mind Blast and feeblemind effect from Ego Whip, and so forth -- all despite drastically simplified mechanics. This sits better with me. I think that the theme of psionic effects being uniquely unnatural (flavor-rich character-wrecking stuff on par with level draining) is even more important from a thematic standpoint than retaining a table-based implementation as psychic arm-wrestling. PX1 has a more mundane and less horrific idea of what psionics should be -- a design preference which might be to the liking of some, but not to others.
There are also a few questions that could have been anticipated and answered a little more directly in the text: Does being hit by a spell or physical attack break the concentration required to activate an ability? What is the "psionic level" of a wildcard user? How are new disciplines and modes learned/selected during character creation or when leveling up?
One final minor complaint is that the rules frequently reference use of the Astral and Ethereal planes, which aren't explained in much detail and may or may not fit easily into the cosmology of a particular campaign world. This is another legacy dating back to the original Eldritch Wizardry version, which packaged psionics together with some new astral/ethereal monsters.
But there's no reason not to buy this product, since it's easy enough to just use the parts you like and ignore the parts you don't. Just be aware that you're purchasing only a conservative revision of one of the more problematic sections of the rules-as-written AD&D experience. I suspect that I'll use the classes and monster stat-blocks on a regular basis; I'm more likely to either write my own system for psionic combat, or to just ignore this section entirely. I'm certainly interested in purchasing a future PX2, perhaps a "Manual of the Planes" with some discussion of astral/ethereal encounters or some alternative visions of how psionics could work differently in other worlds.
(It should be noted that this is a "first impressions" review, and some opinions might shift after playtesting.)