My husband and I took the KTTK tour while on our honeymoon this past January, and it was fabulous!! Rae, our tour guide, was very knowledgeable and funny, and really made everyone feel comfortable throughout the whole trip. You learn all sorts of wonderful "secrets" that really make you understand how Disney pays close attention to detail when creating their magic. You start the tour across from City Hall with coffee and water, and usually have a little time before the tour starts to get some pictures of Main Street basically empty. You choose your lunch off of a menu card (from Columbia Harbour House) and then you do introductions and work your way through some of the lands of Magic Kingdom. You get to ride 2 or 3 rides, depending on how big your group is and how quickly the tour is progressing (we rode Pirates of the Caribbean, the Jungle Cruise, and the Haunted Mansion), and you also get to check out some of the parade prep areas backstage. Towards the end of the tour you get to go down into the Utilidors and look around a bit (we mostly looked at the map of everything while down there). The tour varies depending on the tour guide, but is definitely worth the $58, especially since it includes lunch, and a coveted KTTK pin. Sign up as soon as possible!!!
And definitely check out the allearsnet.com link that someone posted above - there are some great (and detailed) reviews of the tour on there.