Any attempt at trying to actually measure intangibles like personality traits has a lot of slop factor. It is entirely possible that the results will be very insightful. It is also entirely possible that they will be less helpful than most measurement-lovers like to think. I believe the MBTI is useful more often than not, but that often depends on the circs under which it is taken, and who will be interpreting the results.
I think Meyers-Briggs can be useful, but not when it's used to limit rather than to understand, as if there are only 16 ways of being, and once you have a person's four letters, you've got them all figured out.
Sometimes I've found it helpful. E.g. I'm an INTJ or an ENTJ depending on my mood at the time, while DH is an INTP. So, the test says we're a lot alike, which I already knew without its help. But that J-P distinction is important. According to the scheme, I'm a Judger, which means, among other things, I'm happier once my mind is made up. While I like to research and analyze data, I tend to make a snap decision the instant I feel like I have enough info. DH is a Perceiver, which among other things means he's happier when he has several options still open. It's really amazing how many of our arguments are something like this:
Me: Will you make up your MIND?
Him: I need to think things through.
Me: What's to think through? We researched this. See how neatly I summed up the data?
Him: I just need more time.
Meyers-Briggs here is a useful tool because it gives me a shorthand for understanding this particular difference and reminding myself that if I give him more time to digest the data, he WILL eventually come to a decision, and usually one that agrees with the one I made two weeks ago as soon as I had all the data I needed!
However, one thing that's driving me crazy about the New Boss is that she seems to think because we're both INTJs, I will think like her on everything. It's true that we're by far the most organized and focused people in a department of scatterbrained free spirits (chaplaincy isn't a natural field for the focused and goal-oriented), but I'm NOT HER. She expects me to thinks that budgets and business processes and organizational structures are fascinating because she does and I must be like her because I'm an INTJ. And I'm sorry, they're boring as HELL. I save my obsessive focus for things that interest ME.