We did the tour on our last trip and it was wonderful. The tour lasted about 4 hours, and I believe it was somewhere around $40/person (but don't hold me to that!)
The tour included a walk down Main Street, where we were told all about the store windows and what all the names, etc. painted on them mean. Also, a trip of the underground tunnels, and trips on both the Haunted Mansion and the Jungle Cruise to see some hidden mickeys. We also went behind the stores along Main Street to see Tinkerbell's "landing platform" when she flies from the castle before the fireworks each night. We also went behind the gates that are on the left-hand side when you cross over from Adventureland to Frontierland, where we saw the building the Splash is housed in, and since have called the attraction Splash Building...

We were then treated to lunch at the Columbia Harbor House (which is very yummy)
At the end of the tour everyone got a Keys to the Kingdom pin, which can only be obtained if you take the tour. I really did enjoy the tour, but some forewarning...nearly the entire tour was walking (other than when on a ride or having lunch). Although the traveling pace was manageable, one of my family members found the constant moderate-pace walking to be unmanageable, and she ended up leaving the tour after about an hour.