Sometimes, you just get a workhorse of a book. Why does The Will and the Way, by L Richard Baker III, exist? Because Dark Sun decided that a half-baked class from an optional supplement would be a central part of its setting, and thus needed the Complete* series treatment. We get some really basic kits (the Noble Psionicist is a psionicist while being a noble, the Mercenary Psionicist is a psionicist while being a mercenary, and the Psiologist is a psionicist who is good at psionics), some powers that range from the bland, but useful (deflect) to the flavorful, but game-breaking (time travel), and a bunch of background stuff that we could probably have inferred from context (the various city-states each have their own psionic academies that reflect the character of the polis).
But that's really the good thing about having a workhorse - you can put it to work. It's not an exaggeration to say that The Will and the Way is an essential supplement for anyone who wants to play a psionics-heavy Dark Sun game, and at least half the information here is abstract enough that you could port it to basically any setting. No complaints here. . .
Except, it doesn't really solve the essential problem of psionics existing alongside magic. I've said in the past that I think AD&D's psionics system could replace its magic system, and my reasoning is that psionics has so far been mostly a crummier version of magic. Which maybe doesn't sound like much of recommendation, but if you're looking for a class that's competitive with the Thief or the Fighter without relying on an ill-conceived "career balance," then having weaker, more unreliable abilities can be an advantage. If you view them as a peer to Thieves that have a 50% chance of failing to pick a lock or move silently, then their only real flaw is that strong powers like Disintegration and Metamorphosis are not level-gated (instead having a soft level restriction based on your available PSPs).
Where you run into a problem is when you have psionicists in the same setting as mages and priests. Then they get tricky to justify. "Like a wizard, but objectively worse" is not much of a niche. It could be a niche in Dark Sun, where arcane magic destroyed the world and is in the process of using up what little is left, but neither the boxed set, nor the original CPHB, nor The Will and the Way actually bothers to stake out that niche. It's a lesser power, accepted because it does not bear the terrible cost of sorcery, but we never learn what magic can do that psionics can't, nor how people distinguish between them in practice, nor about conflicts and synergies between them (except in Dragon Kings, where 10th level spells require being a 20th level psionicist to use, but by that time in the campaign, it's too late to start asking questions that have been relevant since level 3).
It pains me to say it, but I think what's going on is that Dark Sun is experiencing the weaknesses of its AD&D chassis. Psionics and wizardry and elemental priest magic are all separate phenomena because psionicists and wizards and priests are all separate classes. Your character sheet says "psionicist" and therefor you are axiomatically something different than a character whose sheet says "mage." The people in the setting can instinctively sense this, which is why it's okay for Draj to have The House of the Mind, even though the people would go berserk if someone tried to set up a sorcerous academy.
The thing is, from a setting perspective, this is all very solvable. What you do is restrict yourself to One Weird Thing - on Athas, people have the ability to sense and manipulate life energy to create extraordinary supernatural effects. If you restrict yourself to your own life energy, that's psionics, and it's considered both fair and natural. It's your life energy, so you can do what you want with it. However, the psionic capability isn't purely an internal power. You start by sensing your own energy, but that sense, once opened, can detect the external life energy in the world around you. Likewise, the faculty by which you harness your own internal power can be extended into the world. You grab on to the energy in the air and the earth and the waters, and that becomes sorcery. Go a little farther, and you can take the life energy of other creatures, prematurely withering them, and that's defiling.
It's a continuum, and the boundaries between the magic types are ethical boundaries. Psionics is safest and weakest, you only use what's yours. Preserving magic (aka "priest magic") is just a stronger version of psionics, drawing from deep wells of sustainable energy that are too diffuse to support the grandest of effects. Defiling magic is the strongest psionics, because it is indiscriminate, taking whatever just happens to be available without regard for consequences. Characters in the world can tell the difference between the types (or, at least between purely internal psionics and external psionic sorcery) because everyone on Athas has that psionic sense. They can feel the motion of life energy when it's taken from the world around them (and especially when it is stolen from them personally, by defiling) and when an apparently supernatural effect happens without triggering that sense, they assume the person is being responsible. Therefor, culturally, it only counts as "magic" when it's big.
From a game design perspective, this gets a little tricky, because what you really want is a hybrid class, one that is tempted to transgress the boundaries between the magic types. But you don't necessarily want to sort the types into clear tiers. The quick and dirty way to do it would be: 1st 2nd, 3rd level spells are psionic; 4th, 5th, and 6th are preserving; and 7th, 8th, and 9th are defiling. Except that you kind of want players to be able to be defilers from level 1 and to remain psionicists even at level 20.
Maybe you could refigure the PSP economy. You have a basic pool of PSPs that you can play around with, which increases by level, and so long as you're only using those, you're wielding psionics. But then the more impressive powers have higher PSP costs. You can wait until you're high enough level to use them, or you can call upon a bonus pool of external PSPs. Gathered slowly, these PSPs are preserving magic, and gathered rapidly, they're defiling magic. At various decision points in your class progression, you can select class features that optimize one of the three paths, but you still retain access to each.
It might work, though the game you're playing at that point is barely AD&D any longer. Also, this whole line of speculation is getting pretty far afield of what The Will and the Way actually brings to the table. It introduces some new mechanics to make psionics more competitive with magic, which is maybe not great for the lore, but is helpful in making the classes more balanced. Now you can improve you power scores with extended meditation. This patches one of the biggest weaknesses of the class (your signature abilities fail half the time or more), but at the cost of being extremely tedious. Want that Telekinesis to go from a Wis -3 check to a Wis -0 check (thus from a 60% chance to a 75% chance, assuming a 15 wisdom score), all you have to do is take your character out of commission for 80+ days and succeed at 8 savings throws vs spells (one every 10 days, if you fail 3 in a row you have to start over). In theory, they can also use psionic meditation to improve their base Wisdom score (for psionic purposes only) and get that success chance up to 90%, which is actually pretty decent. But it does require you to not play your character for large stretches of time.
The character tree might make that style possible, but I think I'd just rather have a functional class. The Will and the Way noodles around the edges, but doesn't necessarily make a compelling case for psionics as a whole. What Dark Sun really needs is a dedicated system of its own, something that can handle the psuedo-science fiction of its ubiquitous psychic powers as well as the sword and sorcery implications of its arcane magic. This book is, as I said, a workhorse. It doesn't have that kind of ambition. It will improve your AD&D games, but ultimately, you're still playing AD&D.
Ukss Contribution: I liked a lot of the new powers. My favorite was "molecular bonding" which let you use telekinesis to cold weld/glue inanimate objects together. Imprison someone in their own armor! If I ever play another AD&D psionicist, that's going to be an essential tool in my kit.
However, the idea with the most long-term potential is actually just a concept that keeps showing up - intelligent, psychic undead. The sci-fi version of psionic powers downplays the spiritism that was historically associated with psychic abilities, but I kind of like the idea of a ghost existing because of advanced telepathy, rather than necromancy. I'll have to brew something up.