I actually have quite a few ISTJs in my life, my mother, an uncle and my sister's SO (all tested and confirmed). I suspect three three others (deceased grandfather and two uncle...my GF's sons and my mothers brothers), but cannot be certain. If I had to choose a word to describe them, it would be competent. Give them the right tools and they can do anything. Don't given them the right tools and they will make them. Often I think how amazing it would be to be that innately competent.
While they do not generally select the same fields as I myself would choose, they are experts at what they do, and it never ceases to amaze me. It is quite interesting to watch Si at work. It is a bit like Ni, but in regards to concrete things. All the ones I know say the same things when presented with a problem to be solved...something equating to "let me look at this." Then they think about it with this intense look on their faces and come to catharsis, while I am siting there lost, wondering what they are 'looking' at. Much as they no doubt have no clue what I am staring at out the window during dinner. I assume they are weighing the current situation to past experiences. Fascinating.
The ones I know can at times be a bit more rigid in their thinking than I am, not quite as open-minded. This sometimes brings us into conflict on certain issues. That said, if I am particularly depressed or upset, I have a tendency to be the same. My mother, in particular, who is a very unhealthy ISTJ, is extremely judgmental and negative, to the point of alienating people. It is almost this knee-jerk "I am right and everyone else is wrong if they do not follow this particular line of belief and/or thought." The healthy ones I know are quite well adjusted, funny, and considerate. Their humor is generally a bit more grounded and literal than my own. They seem to find the same sort of release in solitary work that I do in solitary idleness.
One interesting thing I have noted in observing my ENFP sister and her ISTJ SO, is that her Ne tends not to mesh well with his Si, especially regarding concrete tasks. These types are mirrors of on another as far as the functions. He knows what needs to be done, has reasoned it out, and done it many times before, and here comes her brainstorming the situation based on no actual knowledge, but a thousand what ifs. Drives him nuts, the the point of exploding. And his seeming rigidity does the same for her. I am not saying this is true for all situations, only my observations of two individuals. My Fe usually keeps me out of conflict with either, when I want it to.
Again, I speak only of those I personally know, and have tested. I cannot speak for all, or most, ISTJs.