We went to Alcatraz on the 6:30pm night tour. Boarding began at 6:00 but we waited in line for a full 30 minutes to get of the boat. The ferry trip over to the island was nice, but with so many people on the boat at the same time, you couldn't see the sights well or get good pictures.
Once on the island, it takes another 15 minutes to get up to the main prison building and get your audio tour headset. The audio tour was very interesting and we enjoyed waking around the prison. However, there were really too many people in the building to have a good experience. Often there were 10 or more people in front of whatever the audio told you to look at. It was impossible to get any good pictures because of the crowds.
All that said, Alcatraz is something you should see. Perhaps book the earliest tours so that fewer people are on the island. Since there is not really a time limit, people from earlier tours accumulate throughout the day.