I thought I would do this overall post on how I felt about the kiwi experience. I hope this is helpful if you are considering travelling with them or just thinking about going to NZ (which I think everyone should do).
The pros - I have to say I think overall the kiwi experience is a really good way to travel around the country. It is obviously aimed at younger people, but despite this there was a lady who was probably about 50 on our bus! But I would say any age up until 30/35 it is more aimed at you. We had a very strict time schedule, so for us this sort of coach trip was perfect as we went everywhere. Another bonus is that they literally sort everything for you, your hostel, your activities, they sort it. A sign up board goes around the bus every day and you just write your name down. Simple! It is a great way of meeting people, we met sooo many lovely people from all around the world. I think it's especially good if you are travelling alone, as it basically throws you into making friends. I also don't think it's that expensive way of travelling around NZ, as once you've added insurance etc to hiring a car or campervan, I bet it adds up. I would always say buy the pass that's on an offer, we booked the Rangi tour for £353, which was about half price.
The cons - Despite all of the good stuff I've just babbled on about there are some things I would keep in mind when booking. I think the whole hop-on hop-off bus thing is a little misleading. We went at just before peak season so we didn't run into much trouble when booking buses, but just as we were leaving we were meeting people who were finding it difficult to book on buses leaving Auckland! This means that you could get to a place and want to stay a bit longer, but then have to stay for 4/5 days until you can get a seat on a bus. So in this sense I would try and plan where you want to stay as much as you can beforehand, so that you can book your buses.
Overall I would highly recommend Kiwi Experience to anyone. We had the BEST driver who I also feel needs a shout out who made the trip for us! Next stop.....FIJI!