On one hand, we have the Soulknife, who is able to conjure blade weaponry at will and throw them for ranged attacks. On the other, we have the Soulbow, which specializes in throwing attacks, but uses a separate projectile that has its own damage values separate from the blade. Is Soulbow really worth it? Is there really a particular advantage of taking levels in Soulbow beyond being able to apply archery feats to your attacks (e.g. Point Blank Shot and Manyshot), or would it just be better going full Soulknife and taking feats to enhance throwing ability?
Both existing answers cover Soulbow pretty well. It’s fairly mediocre, but it has its schtick. It makes one of the better archers in the game (archery is bizarrely under-supported), and you can achieve a fairly impressive degree of Single Ability Dependency as mentioned by Jeor.
The Soulknife, on the other hand, is awful. The Soulknife’s primary schtick is “I have a magic sword.” That’s what the overwhelming majority of its class features boil down to. The problem with this is everyone else who cares to also has a magic sword. And gets far more options and powerful effects on it, to boot. In addition to their real class features.
Generally speaking, class features should not be things you can just buy. That’s a really bad sign for a class. Sure, spells can be bought (scrolls and wands), but those are limited (maximum spell levels, low Caster Level, etc.) and extremely pricey (particularly if you want to improve that Caster Level), which means you cannot really replicate what a Wizard does by buying items.
A Soulknife, though? Sure, a +9-equivalent weapon is expensive, but by the time a Soulknife has one, so does anyone else who needs one (and they probably have a +10, and with better abilities than a straight +5 enhancement bonus). And unlike your class features, which they just bought, you cannot buy their class features.
The rest of the Soulknife suffers from the same problems as the Monk, which it appears to be largely based on: random assortment of abilities, almost all of which are not worth much at all. Psychic Strike is worth particular mention: this is one of the worst sources of bonus damage in the game:
Knife to the Soul is OK-ish, but the Rogue can do the same thing and more (without all of Psychic Strike’s problems) with the Ambush feats out of Complete Scoundrel.
The main problem with Soulbow is that the projectile is always fired from mundane longbow (Complete Psionic, p36):
While Wis to damage is nice, inability to enchant the bow (be it weapon enchantment, oil, spell or artificer infusion) nor to fire your mind arrow from a physical bow is somehow crippling. Even bonus feats don't really offset this circumstance, which makes Soulbow to perform even worse than plain Soulknife, unless you want to focus on the archery (in which case Soulbow is better).
You also may want to consider Soulbound weapon ACF for the psychic warrior featured in this article. It allows to retain the flavor and to increase performance significantly. At the very least, consider Hidden Talent and Bonus Feats ACF featured in the same article.
If you absolutely must create an archer based on a Soulbow, make him SAD.
The Soulbow is a significant improvement for a Soulknife intent on performing ranged combat. These are the main strengths it has:
The last may not be seen as such a big deal (after all, the Soulknife gets Strength), but functionally, it's one of rather few mechanically good reasons to take levels in either class. It enables you to create nearly Wisdom SAD (Single Attribute Dependent) fighting characters when combined with things like Zen Archery (Complete Warrior) and other things that can expand the use of your Wisdom modifier.
Note that neither class is very good through the end - unless you have something specific in mind (such as constantly creating a supply of Lucky arrows), both Soulknife and Soulbow are best off as dips to support a larger build. That is, if you are building a Soulbow - the Soulknife itself is not good enough to be even dipped on its own merits, in my opinion.