I'm an INFJ male who's dated two INTJ females, and my best friend is an INTJ female.
There has been a lot of good advice so far. I've especially enjoyed the "it was great when we weren't trying to kill each other" comments.
My experience is this, INTJs see the world like we do but they think completely different than we do. This causes an amazing kinship because we're on the same page so often, but can cause epic clashes because we disagree on what we both see so clearly.
Here are a few tips to add to those above:
INTJ females keep their F function in check most of the time, but they are still females, which means their F function is going to pop out from time to time in very large and often uncontrolled bursts because they don't have a lot of skill in wielding it. When their F function comes out, be prepared for nothing resembling rational (to INFJ reasoning) because it is Fi, which is the opposite of our Fe. When it comes out, back off and let them have the floor. It may sound patronizing, but whatever they say or feel in that moment is right (even if we know to the core of our being it is not, and especially if it looks like a personal attack on you) because Fi will be severely antagonized by trying to control, improve, or admonish it. Let her get it out of her system, and I can assure you she'll be right back to rational. Give her as much time as she needs to let it out, which will rarely be more than a few hours, and seldom more than a few days. She'll let you know when she's done because she'll 'go back to normal' and will likely appologize. While she's doing this, it is much better to say you need a moment to think or you're getting too emotional and need a pause for a moment than to just nod and act like you're ignoring her. She really needs to be heard, but will respect you for being upfront. Once you're caught up to her, get back in there and let her continue until she is done. I promise you, it will be worth it. She'll be an amazing partner for a long time after that.
Because she's a female, her elevated F function will almost always work to the advantage of the relationship. However, it is Fi, which means she's going to have trouble seeing how things are supposed to be (according to Fe reasoning). Resist the temptation to rebuke any notions that seem immoral, callous, or self serving before you have a chance to consider them. The biggest advantage of an INFJ / INTJ relationship is the fact that we see clearly in each others' blind spots. Her ability to see into your blind spots comes from her Te/Fi reasoning, and from time to time it is going to seem alien to you. Learn to accept it as the advantage that it is, and she will actually help you become a much better all around person. You'll also begin to see that INTJs are actually very benevolent and moral people... just from a perspective entirely foreign to our own.
But, bear in mind that she's a T dominant girl, so from time to time there are going to be some notably not girly behaviors. Again, this generally works out very well for the INFJ male in almost all cases. However, sometimes it doesn't. For instance, none of my INTJ girls rated hygiene highly, and I had to remind them to shower when they got smelly. In any instance where you have to nudge them into girly behavior, be gentle and polite, even diplomatic if you can. All of my INTJ girls got offended when it was pointed out that they were doing anything especially un-feminine. They really do want to be feminine, and it's not their fault that they sometimes forget. Ni and Te can really distract them from the girly stuff, or at minimum put it on a lesser priority than other things. Learn to accept this early, regardless of how your Fe wants to feel about it, and the relationship will do very well. The upside to this is that Ni + Te often creates a perspective on sex like a guy, making INTJ females very capable and enthusiastic lovers without unrealistic expectations. Seriously, it's more than a fair trade.
My INTJ best friend and I have a funny notion we share. I'm always right and she's always correct. If it is a question of right or wrong, I'm always right. If it is a question of correct or incorrect, she's always correct. Also, we realized that very often we can both be right and correct while disagreeing. Sometimes things that seem mutually exclusive are not. You'll do very well to remember both of these when you disagree. It avoids a lot of epic clashes with us.
Good luck, and I hope it works out. INTJ girls are one of the best pairs for INFJ guys that are possible.
