This is a great tour of a part of Carlsbad Caverns that relatively few people get to see, and it's accessible for most able-bodied adults and older kids (elementary age or older). You walk through part of Carlsbad that leads to the deepest part of the cave, using only candle lanterns for light. There aren't many formations in this part of the cave, though there are some. The big attraction, though, is the history that you get from the ranger and the feeling that people must have had 100 years ago. Caves look a lot different in flickering candle light than they do lit up by bright electric light.
I've been on the tour twice, once in 2010 and this year, and this tour wasn't quite as good - our ranger used her flashlight too much. However, the tour is still definitely worthwhile. It's about a mile of walking round trip, but nothing difficult - largely flat, with only a few places where you need to watch your step.
Definitely a worthwhile tour to take in the cave, especially since it starts at 9AM: you're back by 11, in time for a largely full day, with plenty of time to walk around the Big Room and take the Kings Palace tour. Book early, since the tour fills up quickly - we got the last two spots when we booked 2+ weeks in advance.