I have had the curious experience of living in close quarters with an INTJ, in fact I still do. If 'live' can be so loosely applied.
I missed this the first time I read this thread.

I am involved with an INTJ presently (almost 4 years) and have noticed curious changes. The relationship has transitioned from one of intimacy to one of more semi-detached companionship.
The initial similarities between us revealed themselves as entirely superficial.
The more I reflect upon the beginnings of the relationship, the more I agree with this. What initially drew us together was the ability to communicate and discuss various topics of mutual interest. Over time, as these topics were exhausted, discussions grew further apart and were replaced by more mundane subjects. My explorations grew more varied and information about these explorations, though interesting in themselves to her, were, for the most part, never pursued.
Not that she never explored, rather there were logical extentions from where she was which required exploration. I, on the other hand, would wander down some dark alley because it had attracted my attention.
We cannot, and I doubt we ever will, understand one another. We are two of the world's worst communicators. Attempting to exchange information is like pulling teeth with no anaesthetic.
Again, I see this occuring with greater frequency. Where I will return to a subject with additional information to augment the current status of the subject, she has already defined it to her satisfaction and logged it somewhere. She has acquired a relatively set amount of subjects and anything outside those parameters is inconsequential.
Our libraries differ. Where hers contains books on a set of subjects which interest her and she adds primarily to this set with little deviance, mine contains a wide variety of subject matter to which I am continually adding different, yet related subjects which in turn suggest other subjects. I have no problem relating two or more of these subjects to each other in multiple combinations, whereas, at times, she ses no connnection or relevance between some to others.
Also, I am at the public library constantly
(how many of us know librarians by first name
and know what section to find what subject in?), borrowing books on various subjects, some to read, some to merely peruse, yet she prefers to purchase books at bookstores and invariably haunts only particular sections.
And from my perspective he approaches everything back to front; we can be given the same problem and same facts yet draw the opposite conclusions/solutions.
Ditto, in many cases. Where I see a problem from multiple angles and can conjure various avenues regarding solution, she considers one, and possibly one other, which is optimal and rejects all other possibilities and facts leading to other solutions.
Actually, I find he doesn't want to deal with nearly as many variables as I take into account. He's simply not interested in half my sources of information.
Driving somewhere is an apt analogy. If it is a re-occuring destination, she will usually follow the same route and contend with whatever traffic conditions are present. I will have mentally mapped out several alternate routes which I can choose from depending on multiple variables such as time of day, etc. and can adapt to another route if there are inhibitions to the chosen route.
He stops thinking about something when he has the answer that suits his purposes, I keep thinking.
She has stated on more than one occasion that I "tend to think things to death" and rarely take action unless pressed.
I see the differences as I being much more interested in the theoretical and whatever anomalies arise, where she seeks a solution and implements that solution without hesitation. Additional facts/info rarely deter her resolve.
Though, in contrast, when she becomes stalemated in something, I can view the problem and discover other solutions which circumvent the stalemate.
He is incredibly rigid, my opinions change like the weather.
A rolling stone in contrast to a leaf in the wind.
He wants routine, I wish every day could be an adventure.
Heh

Don Quixote and Sancho Panza. Calvin and Hobbes.
This is probably one of the greater alterations in our relationship. Mutual exploration was evident at first, but now my explorations are more solitary.
hmm. I can't believe I ever thought INTPs and INTJs were similar.
Being relatively new to the investigation of MBTI, though I have read about it and took a couple of tests before, I am comparing the defintions and qualities only now. A few of the differences which seem to hold little significance when in print, are becoming obvious. Some of these differences are becoming barriers.
The one aspect INTP and INTJ share, the need for solitude, also seems to be a dividing factor. We each have 'offices' with copious amounts of computer equipment (who woulda guessed

). She focuses on one or possibly two themes and has multiple windows or tabs concerning them on both screens. In contrast, I have multiple tabs/windows which may initiate at one subject, but carry off into branches and streams into multiple subjects which have piqued my interest and may relate to something I hve explored at some other time in the past which I bring up and relate to that which I am exploring presently.
She will be intensely focused on what is present and concerns her to the point where any distraction is an invasion and an annoyance. Though for me, a distraction may also be an invasion, I must explore that distraction to discover its source.
I think there may be compatabilities on intellectual levels and possibly a few others, but from personal experience, this combination of INTP/INTJ has inherent difficulties which may lead to a reduction in quality of the relationship.