Many tour companies operate half day tours from San Francisco to Muir Woods. I would check into what else the tours offer besides the woods. A tour would be a good idea if you do not like driving slow, curvy roads with steep drop-offs.
If you are more than 2 people, however, I would definitely consider renting a car for the day rather than taking a tour bus. First off, it'll be cheaper. Secondly, a guide is really not necessary for Muir Woods; the brochures do a more than adequate job of explaining the history of the park. Plus, if you are in your own car, you are free to stay as long as you like and explore the hillsides, rather than just having enough time to do the main loop. You can have a very enjoyable full day in southern Marin County (though Muir Woods should only take up half of it, at the most). You can also check out the Marin Headlands, Mt. Tam, and the towns of Sausalito, Mill Valley, and Tiburon. Or, if it's sunny, you can go down to Stinson Beach.
Muir Woods is about 12 miles from downtown San Francisco. There is no public transportation to the monument and its quite an expensive taxi ride. A couple of the tour companies offer half day tours to the woods and then to Sausalito. This is a relatively inexpensive option ($37 for adults as of 2005) and gives you about an hour at each destination. Most tours also make a quick photo stop at the Golden Gate bridge.
It is best to get here early -- both for getting parking and for seeing the forest with as few people as possible.