It gets you places
The Kiwi bus is perfect if you like getting around easily to certain places, like to stay there and then get on. The bus drivers (some more than others) are helpful and will give you information about the different places that you pass by along the way or where you will stop. Also, the option to stay longer in places is a real plus, as it gives you the opportunity to discover a place more in depth. Stops along the way are supermarket stops (sometimes, mostly on the North Island), pictures stops, short walks and toilet stops.
I should say here that I traveled with a friend and that we were mostly in New Zealand for the views, hikes and culture. If you feel more adventurous with sky diving, bungee jumping, etc., Kiwi experience will really suit you as they mostly focus on those activities. One of the bus drivers called me and my friend 'boring' as we didn't do those activities.However, if you feel fine using the Kiwi bus for getting around, that's perfectly fine.
Personally, I felt the Kiwi was okay, but it can improve on some aspects. For example, sometimes you arrive at a place at 2.30 PM, while all activities in that place start around 1.30/ 2 PM. Meaning you can do absolutely nothing anymore in that place. This happened to us in Queenstown. A real shame, if you don't have the luxery to stay a day longer in the place. This is something that can really easily be solved, as the drive itself was only 2 hours or so, but they make it around 5 hours by stopping by a bungee jump place for an hour and then stop a few times more for half an hour for toilet breaks. By just cutting one stop or shorten it, it would allow us to make so much more of our day there.
Also, some activity organisations save space for Kiwi clients, but not all of them. So if you go during summer holidays (as was our vacation) and you hear about the activity in de bus on the way there, chances are that the activities are already fully booked. Unless you feel like staying 2 extra days. This is really a shame, as we counted on some of them but had to cancel due to our schedule. My advise is, if you really want to do an activity, to book it yourself in advance without the Kiwi experience discount. Often it is only about 5 or 10 dollar, so you don't miss that much discount.
Also, the way from Abel Tasman to Franz Josef is split into 3 days / two nights. I feel like it should just be one night. You really lose valuable time here. The second night is designed as a funny night out, with a theme party night for only Kiwi-travelers. As we often stayed an extra night somewhere and had about 8 busdrivers and different people in the bus every time, we found it a bit out of place. It was mostly fun for the people who have traveled with each other from the beginning and were doing the tour in the minimal amount of days. Exchanging this night for an extra night in Wanaka, would be my recommendation.
So in the end, look at what you need and would like to do. We made a list in advance what we wanted to see and what we wanted to visit. Kiwi covered about 90% of those places and the other 10% we could do by staying an extra day or hitchhike. I feel that we did the right thing in choosing Kiwi, as it fitted best with our schedule. However, I do feel that you get most out of New Zealand if you can combine Kiwi with lots extra days and perhaps even an car, as your tour will otherwise feel a bit incomplete.