We did two tours last week, and loved both of them. Our first was Undiscovered Treasures of the World Showcase. I really enjoyed this one. There were a few minutes backstage, but most of it concentrated on the imagineering that went into each of the countries. I found it to be very interesting and enjoyable. You really do get a great understanding of many of the "small things" that are in the World Showcase that frequently get overlooked when you're rushing from ride to ride and show to show.
I've not taken the Segway tour of the World Showcase, so can't provide a personal comparison between the two tours. However, one thing that was pointed out to us by our guide (granted he's a little biased), is that the walking tour has a guide who takes you into buildings and gives you insites on design features, hidden secrets, and the reasons why things are how they are. The segway tour takes you through many of the same areas of the World Showcase, but there's no information provided. The guide is just a leader of a neat drive through Epcot on a segway. Also, he told us to consider that on a Segway, you're basically standing still without moving your feet for two hours. It's actually more physically demanding than the walking tour.
I also really enjoyed the Keys to the Kingdom Tour we took a few days later. I didn't find it boring in the least, and couldn't believe 5 1/2 hours had passed when we were done. Our guide was Steve, and he was absolutely fantastic. Our group was very interactive and gave him lots and lots of questions. Most of the really interesting things he told us came from responses to those questions. So, the more you interact with your guide, the better. There were two things I liked about the Keys to the Kingdom tour over the Hidden Treasures tour. The first is that there was a headset to hear the guide on the KTTKT. This allowed him to continue talking to us even when we were walking through crowded areas and not huddled together in a group. On the Hidden Treasures tour, the guide could pretty much only talk to us when we'd stopped somewhere and had gathered around him. On that tour, there wasn't much talking as we walked from place to place in the World Showcase. The second thing I liked about the KTTKT was that we got to go on actual rides. During these rides (except for the Haunted Mansion), the guide continued to talk to us and give us behind-the-scenes details over our headsets. One of the neat perspectives was that our guide, Steve, had previously worked in several of the rides he was giving us tours of before becoming a "vest," and was able to give us interesting tidbits and stories from that perspective, as well. Lunch is provided on the KTTKT at Columbia Harbour House. During lunch, several guides sat with us at our tables and we continued to discuss the Magic Kingdom.
Before both tours, I was a little nervous about two things: that I'd get too hot and tired from walking, and that I'd need to use the restroom on the tour. The guides on both tours took these factors into consideration, and knew lots of places where we could stop and sit in the shade or air conditioning while we talked. There were also ample "biological opportunities" as they called them throughout the tours.
One thing to keep in mind if you're thinking about surprising someone with a behind-the-scenes tour: make sure they're willing to take such a tour. There are some people who specifically don't want to take a tour that goes backstage because they don't want any aspect of the magic destroyed. (I didn't find that to be the case, but others might.)
Also, go into the tours with a realistic understanding of what they are. On the Keys to the Kingdom Tour, you do indeed get to see the utilidores. This is, however, only about 30 minutes of a 5 1/2 hour tour though.
If you have a Disney Visa, be sure to ask for the discount, which is 20% (at least on some of the tours). We just booked the Backstage Magic for 10/3...wasn't sure there would be any openings, since it's so close to time, but there was! I can't wait...we did KTTK last year, and really loved it, so I have pretty high expectations for this tour! 
