I took differential equations from a semi-retired Harvard PhD (this was in rural Washington State, so it was kind of a big deal) at my local podunk community college. He was also great, and started a math club for all interested students, where we'd sit around and work through high-level math problems and read math papers. He did this during his free time because he really loved math.
Some of the professors there were a little zany, and had questionable teaching credentials ("Oh! You're a local businessman? Come teach a course on business!"), but the enthusiasm and... interestingness... of the education hasn't really been paralleled in high school, 4-year college, or grad school, in my experience.