Definitely worth it. You get taken around in groups (there were 8 people in our group) and the guides are incredibly knowledgable. They only have a limited number of times they do the tour (3pm weekdays, 10am and 1pm weekends, and kids under 10s tour only at weekends 9am), but I guess if you were already in the area, the times may not be a problem.
When we visited, I had some discount vouchers from Groupon that gave us a 50% discount on the price, so worth subscribing for these offers. Also to note it's not the easiest of places to find, so leave a few extra minutes in case you miss the turning!!