I relate to both the ISFP and the ISTP. At bestfit, I am definitely of the improviser temperament, but I have more of a chart the course interaction style of the ISTP than the behind-the-scenes of the ISFP. (An INFP friend of mine was convinced I was a 'J' because of my tendency to express my opinion bluntly as well as my tendency to take over in a lot of situations. I HATE being told what to do, and I hate schlepping around, waiting for someone to make a decision.)
I would say, yeah, I'm an ISTP, except this profound mechanical/technical skill bit- well, I can't relate to that. In film school (I was a film major), my main problem was the technical aspect. I didn't want to think about the technical, I just wanted to take the camera and do whatever and see what the result would be- very experiential, trial by fire. (But isn't that an SP thing? vs an extraverted thinking thing?) Anyway, I hate video games, I hate math, but I do love tearing down walls and building stuff and pruning trees, etc.
Also I like theories that have practical applications, like the personality theory stuff. I think I've hung onto this for so long because I, well, had a mental breakdown of sorts a couple of years ago and I am trying to find out as much as possible about myself so: 1. It won't happen again and 2. I can take this practical knowledge and correct myself and live up to my potential, because I have become a seriously lazy slacker, which I wasn't really before. Also, 3, I want to eventually get off meds because I suspect that they are part of the reason I have become a seriously lazy slacker. I used to always be moving, always doing stuff. Now I just sit in front of the computer and read about personality theory.
So... ISFP because I like film, words. ISTP because I am more matter of fact and analytical- always researching, studying, learning. Both because I feel a deep connection to the natural world vs the abstract.