Staying in the Earthship is most certainly a wonderful experience but should be approached as an adventure with an open mind to an alternative way of living. Do not expect a sterile five star hotel - you need to understand the principals of living off-grid and the habitat of an Earthship before booking this accomodation.
I was fully aware of what I was going into and enjoyed experiencing living in a completely new way - including using the outdoor composting toilet, having to build a fire to heat the water for the shower and sharing the house with a tiny jungle of plants and a few bugs, chickens and cats.
Brian and his family are friendly, fascinating and welcoming. I stayed for a week and time seemed to fly by.
Tip: If you do stay, make sure you do the Hakarimatas walk and go see the glow worm valley at night :)
I think it is wonderful that experiences like this can be booked through AirBNB - In my opinion you would not be able to find anything quite like it in the whole of New Zealand.