I've just spent a month travelling around New Zealand in the company of my kiwi girlfriend and three guidebooks: the latest Rough Guide, the latest Footprint guide and ten year old copies of the Mobil Guides to the North and South Islands. Ignore comments about old editions of the Rough Guide; the latest edition of the RG was simply the best overall handbook.
The RG is more comprehensive, better written and more entertaining than the Footprint guide (see, for example, the RG's obsession with t-shirts in Queenstown). The RG has more maps, in greater detail, and with its new format the information is better presented in a more robustly bound volume. By the end of the trip the Footprint stayed in the boot whilst the RG was in the glovebox.
We agreed with most of the observations about different places to visit, in particular which were good and which should be avoided, and the authors have provided comprehensive information about activities and accommodation. The context sections on the history of and writings about NZ are more comprehensive that other guidebooks to NZ I've seen, and there is good information about tramps (walks) and adventure sports as well as the regular info on accommodation, restaurants etc.
Also, it's not the Lonely Planet guide. I'm a great fan of the LP series and they are always my first choice, but in a destination as popular as NZ you will soon find yourself going to the same accommodation and restaurants as everyone else. For example, in one lodge we stayed at in Hokatika a questionnaire showed that ten people had read about it in the LP and just three in the Rough Guide - I expect that's a fair reflection of the number of people using each guidebook. Stay off the LP trail and you'll probably find it easier to obtain accommodation and get a table in a restaurant. Of course that's not a good enough reason to buy a book, just because it's not as popular, but it could make a difference.
Worthy mention should also go to the Mobil guidebooks by Jeremy and Diana Pope - and this may be the best place to do it as they are long out of print. These two guidebooks are designed for those touring the country in a car or campervan, providing a detailed history for each major or minor settlement of interest, as well as sights to look out for on the road in between. Although of no use in finding somewhere to stay or drink, they are invaluable if you're interested in anything from the Maori wars to the formation of the Moeraki boulders.
So, after 5,000km it was clear that our ideal combination of guidebooks is the Rough Guide to New Zealand for all of your travel needs, and (if you can find them) old copies of the Mobil Guides to explain all of the wonderful sights you'll see. Have a great trip - and leave the footprint guide at home!