dissenting opinion: It's easy to teach an interesting course when you get to handpick all of the material. 251 is called "Great Theoretical ideas of Computer Science" because it covers many of them. Alternatively, it's hard to come in and blow your students away with matrix algebra day after day.
His other popular class is called "Science of the Web," which is similarly open-ended.
He's undoubtedly a great teacher, but the other professors are also very competent; albeit in less sexy topics.
small counterpoint to your counterpoint: I had him for 15-381 (Intro AI for non-CMU people), it was his first time teaching the class, and his lectures were substantially better than those of the other professor he co-taught with and had been teaching the class for many semesters. He was either my favorite or second favorite CS prof at CMU.
His other popular class is called "Science of the Web," which is similarly open-ended.
He's undoubtedly a great teacher, but the other professors are also very competent; albeit in less sexy topics.