The Kiwi bus is very popular with British teens on their gap yahs, and I had pretty much nothing in common with 60% of people on the buses I hopped onto. But, I made some incredible friends and had one of the best times of my life.
It's a very slick operation, and all the drivers know exactly how it works, and can recommend places that are best to hop off if you have spare time to explore more. As I did it in autumn/ winter, there were four buses a week, so hopping off usually meant three nights in one place, but in summer there are buses every day. However, it's much busier in summer, and there might be some difficulty getting the bus you want unless you call or email the office and book a few days in advance. Most buses I was on, except the first one with the newbies all starting in Auckland, were quiet. Once you're on one bus they assume you're staying on it and following the set itinerary until you book a different bus with the main office. The timetables are available online.
Most people, including me, hop off for a while in Queenstown, mainly because it's a fantastic place with loads to do. And the most popular route seemed to be Auckland to Christchurch, which works really well. That way you don't have the dull activity-less ride north to Auckland, but having said that I made my own fun on that section of the trip in Taupo and Auckland. The only thing that is a shame to miss out on if you finish in Christchurch is the whale-watching or dolphin swimming in Kaikoura, but you can pay the Kiwi Experience driver NZ$20 extra to take you there, and then arrange a local bus back to Christchurch.
Before every stop you make, the driver will pass round sign-up sheets for activities on offer. These are normally at a good discount, often as good as 30% off. And there are often a variety of options. Or you can do nothing if you want to save money or feel like having an afternoon/morning off. I was in need of a break after five days straight of at least one big activity per day. I loved the madness, but it's nice to sit back from the fast pace once in a while.
The drivers do the same with accommodation sheets. Most people stay together, but if you've booked ahead somewhere else they can normally drop you there. The accommodation can be cheaper if you book ahead online, so if you can find out what hostel everyone is staying at, you can save money that way.
When it comes to choosing what pass you want to buy (basically what you want to see in New Zealand), I would definitely recommend going for one that includes both islands if your time limit/ budget allows. Both islands are packed full of things to do, with the south being more rugged and beautifully wild, and the north full of culture, beaches and activities.
The Deep South (Dunedin, the Catlins, Invercargill and Milford Sound) is very scenic. But if you're pressed for time I would recommend just doing the Milford Sound day trip from Queenstown.
I can't comment on the Bay of Islands section because it wasn't included in my pass, but the people who went there, even in autumn, said that it was beautiful. I'd be more inclined to go and see its beaches if it was summer though.
Either of those sections can be bought from your driver if you want to do them and they're not included in your pass.
Absolutely look at what passes they have on offer at the time you book. This is just one of their pages on their website. I got my Auckland-Auckland Chilly Bin pass for 46% off, bringing it to less than NZ$900. And that includes the cruise at Milford Sound as part of the Deep South.
I didn't meet anyone who said they had paid full price for their ticket. So have a look and see what's on offer. They rotate the passes on offer every so often.
To be honest, I thought that the price I paid was a good deal, and made it worth it. I wouldn't have been happy to pay upwards of NZ$1000 for it. But it all depends on your budget.
And when it comes to deciding how much time to allow, have a look at the 'minimum days' they suggest for each pass, and go from there. Try to leave yourself some days to play with, so you can hop off if you want to. You might meet people you want to stay with. Most people stay extra nights in Wellington, Kaiteriteri and Queenstown. I heartily recommend QT out of those though, if you only have time to hop off in one place. But for the most part I found people didn't hop off in many places, if at all.
Overall, I think the Kiwi Experience is a fantastic way to meet people, have a nice easy time, and see this amazing country. But it's definitely not the only way to do it.