Thousands of people have attended Edward Tufte's one-day course on data graphics. Robert Kosara did not like it.
My advice? Buy his books. Read them. They're good. Just realize that you're getting a historical perspective on data visualization, not the cutting edge. Understand that Tufte's ideas are a good starting point, not a religion. There are many things that Tufte doesn't know, including pretty much anything related to visual perception and cognition, recent work (less than 30 years old), and interaction.
I've never been, but that's sort of what I expected. Has anyone had a different experience with the course?
Update: Lots of good stuff in the comments. The consensus seems to be good/great for beginners, and others should stick to the books for refreshers.