I’m of the impression that the MBTI tends to be more useful to more you tend to the extremes of the four scales. When I do the test, I usually use one that gives you percentages for each scale. I remember that for my first time it got INTP with as percentages (roughly) 100% (I), 80% (both N & T) and P (60%). I was 20 when I first tested. 7 years later I still test as INTP.

I remember that when I first tested I was rather skeptical and knew of the Forer/Barnum effect so to be sure I read all the descriptions of the other personality types. The INTP profile still fitted best. I could also still see parts of myself in profiles that were ‘closer’ to mine (i.e. less different letters), the INTJ one for example (while my exact opposite (ESFJ) was like reading the description of an alien).

On the other hand, I'm still skeptical. Both the creators of MBTI had no psychological degree and it’s scientifically unfounded. There’s also the partly valid criticism that the test just reflects your answers and so is no better than cold reading.

In the end, I took the INTP profile with some bucket loads of salt but still used it to explore some ideas about my own psychology. For example, one of the INTP aspects that struck a chord with me is the switching between a logical mindset and intuitive free-associating goofing off mindset.