I have found it to be real eye-opener recently. I discovered my mother is INFP (Mediator) and my father is ISFJ (Defender). I also asked my group of old school friends to take the test and we all have different personalities, which wasn't surprising. We have the Debater, Entertainer, Mediator, Protagonist, and myself. I've had so many "aha" moments. The way they've behaved in the past and how we've failed to get along on occasions seems to make sense now! I've always felt misunderstood in my friend group so, hopefully, this has encouraged them to learn about INFJ. I feel like I have an understanding of them all now, and I can stop judging them so harshly. The process has literally brought me to tears, particularly reading about INFJ and our interactions with other personalities and the world in general. I'm sure that what I've learnt will be a big help to me in the future.
I have one negative opinion about it, however. I have been dating a douche bag and allowing him to treat me badly because he told me he was an INTJ! I was using his MBTI as an excuse, and continued to see him longer than I should have done. I'll have to be wary of this in the future.
I have also enjoyed learning about Enneagram personality types. I feel it provides more depth to personality, as it further divides the MBTI types. Although, they are rather more negative than the MBTIs! I came out as 6w5 (the Loyalist) and it's made me realise why I've been feeling stagnant in life (work and relationships). Apparently we struggle to trust others and are reluctant to take risks!