The Enneagram appears to be a hybrid of older traditions (Seven deadly sins, Kaballah, Sufi, Chaldean Seal, Gnosticism) based largely on intuition (in an unscientific era). Claudio Naranjo westernized it, and most current authors owe much of their work to his ideas. As a nine trait personality system, I have found the Enneagram definitely has some descriptive value (as I think any personality system with sufficiently distinct traits does). There is still some variance among the trait descriptions from author to author and since the Enneagram lacks a clear structure it will always be hobbled by which description is more accurate. There are also overlap / orthogonality issues, i.e. some Enneagram types correlate with each other.
Moving beyond the Enneagram as just a nine trait personality model is where you encounter a lot of ideas, theories, concepts which are inaccurate, vague, or entirely baseless. The idea of nine traits interconnected on a geometric star shaped Enneagram symbol lacks any empirical evidence at all. I have found the connections points to not fit with attempts to empirically correlate them. Furthermore the ordering of the types on the Enneagram circle is not consistent with how the traits actually correlate statistically. Consequently, wing theory is without basis which is why I am revising all my tests to only give main type.
Variants remain pretty vague and undeveloped in most Enneagram literature. They in theory explain the possible differences of people who have the same main type, but they share the same variance in accuracy of main type descriptions (which, again, lacking a coherent underlying structure makes accuracy resolution impossible).
The Enneagram like the Kabballah is a trendy system which some people are using opportunistically, without regard to the scientific method (which is the foundation of modern society and intellectual progress). Some Enneagram authors and teachers are more money driven than others but the Enneagram community in general tends to be unscientific and new agey, with a boutique aura about it.
The problem inherent in the Enneagram is basically unempirical vagary. In so far as human tradition has a history of embracing unempirical vagary as truth, there is always a potential market for this. The future of the Enneagram beyond nine inconsistent personality type descriptions is marginal unless reproducable evidence can be shown that the types have some relationship implied by the Enneagram symbol (all my research suggests there is none). None of the leading Enneagram authors appear to be doing any research or justifing their work empirically, either due to laziness, ignorance or a realization that much of what they make money off of or have a personal belief investment in does not make any sense when examined objectively. The Enneagram system is, I think, a primative personality theory which in it's current form has no future as a useful personality model.
For more info on the Enneagram, books I would recommend...
Claudio Naranjo - Character and Neurosis (most scientifically based version of the Enneagram)
Helen Palmer - The Enneagram
Don Riso - Wisdom of the Enneagram (most prolific Enneagram author but also the most money driven, and not scientific).
Personality types are not system specific, so even if the overall Enneagram system is invalid or too flawed, the nine types (or more) - depending on whether the differing descriptions from different authors amount to additional types - are still useful in understanding personality structure in general.