I've been to a Tour stage 4 times and a Vuelta stage twice. As other posters have said, the mountain stages in the Tour are fantastic to attend. So many people and an atmosphere that you just have to witness to believe it. I was at Alpe d'Huez one year - epic. Time trials are awesome too. Went to a team time trial in the Vuelta, that was just unbelievable. I can't quite explain it but there's something that just brings tears to your eyes when these guys zip by at 60 K an hour riding so close together their wheels are almost touching.
The Tour of California is nothing like that. But years ago they used to have the San Francisco Grand Prix which was a 15-lap race on some of the steepest inclines in the city. The first year the organizers hadn't put up barriers on the steepest section of Taylor St (18% incline), I guess they just figured the spectators would stay on the sidewalks. They didn't, and the crowd just grew and grew with every lap to the point where the service cars had to stop and wait for police escorts up the hill. On the last lap, Hincapie attacked right in front of where I stood and there was a roar as people closed in on the riders in the center of the roadway and made it as insane as what you see in the Pyrenees. The next year the barriers came up and there were cops out in force. I don't know why the event died, it was said to draw 600K spectators. But the logistics were pretty daunting so maybe that was it.