It's worth taking for fun, and I know some people who've gotten 100% matches into the field they wanted to do, but personally I wouldn't put much weight into the results. When I was a medical student I got general surgery as my top outcome every time, despite the fact that it was one of my least-favorite rotations.
I agree that experiences on rotations probably carries the most weight, for better or for worse. I enjoyed a lot of OBGYN, but that doesn't matter when a third of patients wouldn't even talk to me. The quiz might say I have a high preference for general surgery, but I also have a very high preference for NOT getting shouted at for 120 hours a week. I ended up picking the specialty on which rotation I was the least bored and which had residents/attendings I liked the most, even though it was never high on my quiz rankings.