The audio walking tour of Alcatraz is well worth your time and money. My first bit of advice is to get your tickets early (a couple of days ahead for a weekend). The boats leave every half-hour or so, but they fill up extremely fast. There is a trip to the island itself is about 10 minutes. When you arrive, you are given a brief overview and then set free. You have as much time as needed on the island.
The audio tour is part storytelling and part tour guide. There is lots of good info and stories to keep you entertained.
You can explore most of the island on your own, with a few places off limits (but not that many).
This isn't a tour you will do every time you are in San Fran, but it is a must at least once!