I'm a psychologist and was trained ( a long time ago ) in all the usual stuff regarding test construction. My view--and most psychologist's view, I think--is that the MMPI and all of its variants represent the gold standard in test construction. The idea of the MMPI is that responses to test items, or groups of test items can be shown to be correlated with certain diagnostic groups. The content of the test items doesn't matter. It might be a true/false item as to whether the respondent likes spam canned meat. If it has been demonstrated that most people identified as depressed answered "true", then the item can be seen to have been answered in a way that depressed people answered. The obtained profile, then, can be said to be consistent with how depressed people responded. The test yields a bunch of such findings on a number of clinical scales that, taken together, paint a diagnostic picture. Now, there can be a problem in constructing a reliable sample of homogeneous depressed (or anxious, or antisocial, or psychotic, or OCD) upon which the test is standardized. There are other potential flaws in test construction--too many to list. One flaw, not so much of test construction, is of course, the fact that the test is entirely self-report. But, still, the MMPI has been subjected to so many inquiries, has been the focus of so many studies, not to mention hundreds of doctoral dissertations, that its reliability and validity have been satisfactorily demonstrated (my opinion). It's validity scales are also pretty interesting and have been demonstrated to be difficult to defeat. Now...in my own practice, though, I find that I'm a thorough enough interviewer that I rarely find out anything from standardized testing that I don't observe or haven't already figured out. I was extensively trained in projective testing and never use those instruments anymore. However, I'll use the current version of the MMPI if I'm doing a higher risk eval and want some convergent, objective data to supplement my own findings. It's the only instrument I bother to use in practice. Personally, I think all of the other personality tests--with the possible exception of the Millon tests--are silly wastes of time, no better than horoscopes.