Interesting place. I highly recommend taking the "night tour" after dark, which usually includes several areas of the prison daytime tours don't usually cover. I've heard that they allow larger groups (ie Scouts or school tours) to spend the night there, in cells, in exchange for volunteer service. Don't know if that's still true, but if so, sounds like a really cool idea!
Great place to go visit. Take the little cruise boat out to the island. Then they give you a tour of the island.
This is a must stop if you're visiting SF. They often have special event tours so there are new things to see even if you've been before. The audio tour is awesome and the views from the ferry are great too.
Must do on any tourist trip to SF. Get tickets online well in advance or be prepared to stand in line starting at 5am.