I think ISTJs are some of the best people I've ever met. I've never met an ISTJ I didn't like. I see a lot of similarities between me and them. It's like having your weaknesses thrown in your face for both parties, only it's not offensive and it's more of a learning experience. As for the actual friendships I've had/have they have been very loyal to me and have a pretty high capacity to care when they actually do care, but they won't really show you this.
I just like the concreteness I've experienced. I never think I'm being decieved when I associate with one. They are also some of the more interesting people I've talked to, being near opposite but making the differences tolerable due to the fact that we are both introverts. I also like the whole planning thing that some people have a problem with, with some of my P type friends I will get randomly blown off the day before we planned to visit, but never really when I hang out or do anything with an ISTJ.
I also feel supported by ISTJs. When I am mad about something they usually get mad with me, and point out things that they dislike about the thing I am angry etc about. I also feel like when I am happy about something they will point out the things that they like about the thing I am happy about.
Show some love! ISTJs are super underrated, especially on this board. I haven't had anything but positive experiences with them except in a few circumstances, but these were from unhealthy people.