If you're into this sort of thing, then I would recommend it. I drove out there on my last day in Tucson. It's out in the middle of nowhere. It was interesting to see the sphere itself, but unless you spend the money, you can't get inside to see everything. Most of the time was spent walking around looking at the outside.
Unique Suggestions: It might be worth it to spend the extra money for the whole experience.
I went on the "Under the Glass" tour (not cheap at 22 dollars) and I had to hear the tour guide complain about how nobody wants to invest in using the facilities and how some universities had pulled out of a contract... yawn! I find it hard to sympathize with the owner... besides, I went to see the scientific side of the building, not its financial woes.
They leave you halfway through the tour underneath one of the "lungs" and tell you: "The rest is a self guided tour". Really, really boring and not that interesting... the "ocean" ecosystem is no more than a big fish tank. I think I saw three fishes total.
Fun Alternatives: Save yourself some money and go to the Desert Museum.
Better yet, go to some other city altogether. People in Tucson is not friendly, helpful or even appreciative of the fact that you are spending some money in their funky little town.
Biosphere 2 is a poorly maintained monument to once great and groundbreaking experiment. The tour was lackluster and the "displays" in need of updating. The ticket price is high for what you get.
Unique Suggestions: Don't go during the summer. Biosphere 2 doesn't have air conditioning and its location in the Arizona desert does not make for a comfortable tour. Do enjoy the natural landscape and beautiful Catalina Mountain views.
Fun Alternatives: If you're interested in the science and story behind Biosphere 2, do yourself a favor and enjoy it from the comfort of your own couch. I read this book, The Human Experiment: Two Years and Twenty Minutes Inside Biosphere 2 (available on amazon.com) written by one of the original crewmembers, Jane Poynter (www.janepoynter.com) that was much more revealing about the experiment than a tour of the building.
This was one of the best tours I've ever been on, and I've been on lots of them. I remembered hearing about Biosphere 2 years ago, but learned so much about the experiments on this tour. The facility is incredible - the ocean, the rain forest. There isn't anything like it anywhere else in the WORLD. The idealism of the people who dreamed, and created, this place is awesome. I felt the tour was well worth the $20 I spent. In fact, the visit to the South Lung building was, in itself, worth the money. To see the incredible diaphragm that provides air circulation for the complex, to be in that one-of-a-kind place, was amazing. If you have any interest in science, ecology, or the environment - or in seeing an example of dreams come true - don't miss it.
Unique Suggestions: I visited in January, during the winter, and yet, was glad that our time in the rain forest (at 75% relative humidity) was limited. I can't imagine how uncomfortable it would be in the summertime. Don't try to visit this place in the summertime.
This tour requires a walk of maybe two miles, all together. Be sure everyone in your party can walk that far.