Detailed review by Bloodnok
Make sure you book tickets some time in advance in high season. Access from Pier 41 via a short boat trip.
Also make sure you opt for the Audio Tour. This really brings the entire place to life. All the background noise you hear will be the "prison life" sound effects - removing earphones reveals an eerily quiet atmosphere as everyone is avidly listening to their commentaries. Highly informative, well scheduled and interesting content.
It's one of those tours that makes you say "Wow, I'm really glad I went on that!", and not being easily impressed, that's a real recommendation from me. It really is a "must do" if you are visiting San Francisco.