I saw Rush at their peak in the mid-70's, and their performance of 2112 live was one of my favorite live music performances ever. The show I saw was in Japan, and they had a quad-like sound system set up, so the music played from 360 degrees, which produced some great effects.
Check out this live version of 2112 on YouTube:
2112, Part 1
2112, Part 2
2112, more recent concert footage - seems like the crowd was into it
I still get goosebumps at about 3:05 in on the first part when the lead riff kicks in...
ADG