DD took Fast Paced Biology last summer at the Johns Hopkins campus, and will be taking Neuroscience this coming summer since that's what she thinks she wants to be. Note that while some classes like Math are self-paced, others like Biology are lecture classes.
Our town has a highly competitive public school system (average SAT score >1900), and DD is taking all honors courses, so her classmates are gifted as well. But she found CTY to be on an entirely different level. It was like everyone there was in the top 1% of her 9th grade class. The material was covered very quickly, but students were able to keep up.
DD said she was surprised that everyone there was really smart, including people who just didn't seem to be at first. The other thing that surprised her was that almost everyone was quite nice.
As you can tell, she had a great time. If anyone has other questions, just let me know. Feel free to PM me as well.