I absolutely love the tours at WDW. I've done:
-Keys to the Kingdom 3x (#4 coming up in July)
-Backstage Magic
-The Undiscovered Future World
-Behind the seeds 6x
Here's my take on them, sorry if it goes on too long.
Each has a very different audience to it as well as different information distributed.
Keys happens to be my favorite, it focuses mainly on the Disney approach towards entertainment as expressed through MK. You'll hear alot about Disney philosophies on customer service and get to see the design of the park through new eyes. Also, from what I've experienced, the Keys tourguides are the stars of Disney Cast Members and have the most wonderful personalities. Its primarily walking with about 30% of the tour spent standing still (at least on my tours).
Backstage Magic is somewhat the "ultimate backstage experience", it is 7 hours of tour, but a large chunk of that is spent on a bus or standing still in various areas. (Lunch was just over an hour at Mama Melrose's) The focus on this tour is more of how Disney pulls off such an elaborate show each and every day and just how huge the operation really is. You'd never think about it, but the stop at the Disney laundry facilities is probably more impressive than the utilidor. I would say that the overlap in material with Keys is very low, probably under 20%.
Undiscovered future world was very interesting, but a little disappointing. Our guide was excellent (Thanks Octavio) and it was definitely Epcot to the core, but not as much the technology and innovation side of Epcot as much as a few fun factoids and alot of emphasis about what Epcot represents as an international gathering place. The last part of the tour is a meeting with a foreign cast member who is an ambassador of their country. Really a wonderful tour (and it includes VIP passes to Illuminations) but I think it needs to be renamed or reworked to focus more on futureworld.
Behind the Seeds. Hmm, not much to say. Great value, wonderful "starter tour" and really neat information, plus a bite of free food

I always take advantage of this simply because its a nice break from the usual Disney hustle and bustle.
wow, guess I did ramble on, but I just can't help it, the tours are not for everyone (if the magic gets ruined easily for you) but if they;re your type its worth every penny!