I'm quite near to being ENTP, so I'll also answer:
I know two ISTJs rather good. One of them is really, really clearly an ISTJ, the other one seems to be right on the edge to be an ISTP. While the second one is a very good friend(I always liked STPs), the friendship to the first is a bit irritating. We lead a altar-server-group(hmm that sounds like a sect, but that's in fact quite common in catholic germany; even atheists and agnostics, like me, are often in this groups because altar serving really is the least important thing there) together, so we have to get along with each other. It's not that I don't like him, but we usually don't quite understand each other. He always refers to doing something "how it has ever been done", is very shy around strangers(what he shows by seeming grumpy), and even to friends, he usually changes between being very grumpy and very hyper/goofy. He is just very un-visionary and always takes things waaay too serious(If he would dictate what we do when meeting with our group, we would play football EVERY SINGLE TIME; It's always on me to manage "revolutionary" stuff, like, watching videos or playing board games). I, on the other hand, are in his eyes too ham-fisted/impractical and don't take my work seriously enough.