Its been REALLY scattershot for me..... I've had all ranges (from inspiring to downright incompetent) teachers in all subjects. My personal opinion is that the people who WANT to teach end up making relatively good teachers--no matter the subject. Enthusiasm and the desire to do the job really makes a big difference. If they "ended up" being a high school ____ teacher because it was a "decent job" with their degree, then they usually aren't too great about it. Now if they always wanted to teach high school ____, then they're usually pretty good--they're doing what they wanted to with life.
Sort of the same deal with profs, too--the ones who WANTED to teach, usually do alright. The ones who just want to do research.... well, that's a different story.... and one that's far too long to get into.