My epiphany from first learning about my MBTI type years ago was a confirmation that people really did think differently than I did. I knew that instinctively, but never understood why. Finding out that I was in a box in the top right corner of the MBTI grid and that most of the people I interacted with at work and in my personal life were wayyyyy over in the opposite corners was one of those lightbulb moments for me.
I don't want to fall into the trap of treating people like stereotypes instead of individuals so I don't type people I know. I know that seems odd for someone who hangs out on typology forums, but I really don't care what MBTI type someone is. It's enough for me to know that most people think differently than me and I rely on my own read of someone to figure out how best to communicate with them. I am interested in personality theory and I assume my knowledge of it gets applied when I'm reading people and situations, but I use a lot of other data and skills I've honed over the years as well. I've only ever asked my best friend and my husband to take the test.
I have explained my MBTI type to a few people in an effort to help them understand me. Usually I show them the Compleat Idiot's Guide to the INTJ, it's simple and amusing and gives them some idea of how my mind works. If people are going to stick me in a box anyway, I figure it's better if they use the right one.