I've done Keys to the Kingdom, UnDiscovered Future World, and Gardens of the World.
Keys to the Kingdom was the best of the 3. It was 4 1/2 hours of touring the Magic Kingdom, riding a couple of attractions with the guide pointing out all sorts of stuff, and several stops backstage. It included lunch at Columbia Harbor House. I would definitely recommend it.
UnDiscovered Future World was OK, but less than hoped for. Some of that may have been because it was my third tour in 3 days and I really didn't need a lesson on Walt's life again. The first 2 hours were a bit dull - backstage stops were corporate lounges in SSE, the Land, and Imagination. Then it got better - sneak peek at Soarin's construction site (just a peek), into the control room at Mission:Space, a ride on Test Track via the exit (no wait), and then back to cast member services, wardrobe, and a talk with a member of the international program. It ended with seeing the docks where they load the fireworks for Illuminations. There was a short snack break at The Land (on your own dime) and free water during the international program session. This tour probably has had some changes with all the new stuff going into Future World.
The Gardens tour was great because I went during the Flower show. I'm not a big gardener, but nothing was over my head. Got to see how they make the topiaries, secrets about other plantings around FW & WS, and to see the backside of Canada's mountain. Pretty neat.
This year I'm trying the Segway tour and Mickey's Magical Milestones. I'd love to do Backstage Magic, but I haven't been willing to spend a whole day on it yet, let alone $200.