My fiance and I did this tour back on New Year's Day '09. I had wanted to book the KttK durring that trip but it was booked up. So, wanting to do the tour I decided to book the Backstage Safari instead. I did it more for her than for me, but I was pleasantly surprised. I wouldn't want to give anything away, but it was a great tour and well worth the time and money. We did end up going on KttK on a different trip (it was also fantastic). Just keep in mind that the minimum age is 16 and they are strict about it.
Since you are staying at AKL, did you look into the Wanyama Safari or Sunset Safari at all? We did this in October and it was one of the best experiences we have had. $190 sounds like it is a lot of money for a tour, but it is well worth it. You really get up close and personal with the animals (a giraffe stuck its head into our safari vehicle) and the guide works on the grounds and can give you a lot of information about each species as well as in some cases individual animal habbits. After exploring all 4 of the savannahs you get a fantastic feast at Jiko. The service is top notch, the chef will talk about each of the breads and spreads, appitizers, entrees and desserts (the meal starts with the bread service, then 3 appitizers are served family style, then the pallet cleansor, then 3 entrees are served family style, then coffee and 3 desserts are served to each person), and that doesn't mention the wine pairings that are included (as well as the tip). If you can swing the $190 per person, I would highly suggest it even more than the Backstage Safari. I would also suggest doing it earlier in the trip this way you know what you are looking at durring your stay. The Wanyama is available to anyone, over the age of 8, staying at AKL (Jambo or Kidani).
If you happen to be staying Club Level at AKL, you also have the chance to go on the Sunrise Safari. We also did this back in October. It was ok. Basically you go through KS a lot slower than you would normally. We went through it in about 75 minutes or so when the actual ride durring the day is about 18 minutes. After the safari you go for a decent breakfast buffet at Pizzafari. While it was a fun experience, I liked the Backstage Safari better. The Sunrise Safari is available to all ages staying CL at AKL.