I saw Chicago at Cassell Coliseum on February 27, 1970. They were set up just off one end of the basketball court. All I really remember was watching Peter Cetera sing "25 or 6 to 4" (their hit at the time) and the Terry Kath guitar riff during that song. While playing the song he walked out to the middle of the basketball court (really long cord dragging behind his guitar) and wailed away. Was an impressive sight and sound. I never forgot the name Terry Kath after that. Like many I was heartbroken when I heard he lost his life years later.
"That guy plays the baddest guitar I've ever heard"- Jimi Hendrix (noted in the documentary).