I find the Love Languages test, http://www.5lovelanguages.com/ , despite sounding a bit odd, to be useful. It rates whether you express and feel affection more strongly from Physical Touch, Words of Affirmation, Quality Time, Acts of Service or Gifts.
Mainly useful because while the results made sense, they were not the ones I tend to think of and present as most important (or that I would like to be most important to me) - so they gave me a bit of insight into what emotionally felt best, rather than what I would like to appreciate most. Similarly it's useful to have partners try it, as it may be different from what they ask for or think is important.
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.