I learned linear algebra and spectral graph theory from Babai, and he also taught a notoriously difficult combinatorics class. It was a rare privilege to be taught by someone who was doing brilliant research and also cared deeply about undergraduate students.
I sat in on a couple of those classes. At the beginning of the class there were 50-100 people registered. At the end, there were 6 (including graduate students).
50 -> ~10 students standing was my experience with laci's algorithms class. He's still an excellent instructor, though. Willingness to do hours of background and follow-up reading outside of class was a requirement if you wanted to get a deep understanding of the material, and it pays off well. (He was very approachable though, not at all the research professor stereotype.)
