Have you ever taken a personality quiz/test that helped you have valuable insights? If so, what were the tests and how were they useful?
The only useful ones I've found all yielded the same type of insight. They showed me where I stand relative to others, which is can be genuinely useful since representative samples of large populations can be hard to come by. This includes IQ tests and tests for mental disorders (in my experience, people are usually aware that they are, for example, smarter than the average (although the Dunning-Kruger effect might complicate this) or have some intrusive thoughts and compulsive rituals, but might be surprised to find that they are three standard deviations above the norm or that their symptoms are sufficiently severe to be considered OCD).
No remotely reliable (as in, not astrology) test I have ever seen has revealed genuinely surprising information for a moderately self-aware person, outside of ranking. Furthermore, they rarely gather personality data in a remotely subtle or non-transparent way ("do you like spending lots of time with large groups of people?" "yes..." "surprise, you're an extrovert!"), and thus seem super susceptible to test-takers' attempts to confirm a desired identity.
An example of a more interesting/subtle way to potentially conduct a personality test would be to use question like OKTrends' "do you like beer?" which clusters strongly with "do you have sex on the first date," and, potentially, sexual openness. Such results might be harder for manipulate (consciously or unconsciously) and could assist with deeper self-awareness.
Edited because the first link was broken.
From what I've heard, the Myers-Briggs test is fairly accurate, but even the paid MBTI evangelists say that reading through detailed descriptions of the various types to find the one which feels most familiar is better. The detailed description part is important, because the details (about a page per type, IIRC) go into both the strong and weak points, providing pressure to reject descriptions which have flaws that feel inaccurate.
From what I remember, the corporate paid evangelist visits generally do type testing three ways: Written test, description-based given by your peers, and description-based self-report. The first is for calibration, the third is the main assigner, and the second is basically a conversation-starter; the idea is to discuss ways in which people feel different internally from how they're perceived.