And a very useful one at that.
I see too many correlations and fuzzy boundaries in it. It was originally made in terms of 'fixations', and I am of the opinion that the Enneagram simply covers most of the ways people end up falling into limiting behavioural habits.
I have studied the Enneagram, found areas of improvement, and ultimately have found that there are defense mechanisms and ingrained habits that I recruit from multiple areas within the Enneagram. It makes no clearly discernable pattern, and therefore I think while the Enneagram is useful, its method of categorization is, well, erroneous. Its greatest contribution, I think, is the focus on discerning underlying motivations and habitual patterns to protect oneself.
Hence why some people seem to find that no one type fits properly, whereas others do.