Ask HN: Resources on vitamin D supplements and studies

Hi there, would you share your experience with taking vitamin D supplements and resources & studies you found about the subject? The only good summary of research I've found is from several years ago: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908269/

What did you take it for? Did it work or improve your condition? Any regrets ("not taking it sooner" counts)?