the classic question, i reckon it depends what you are after, kiwi experience is good for a full on experience but does carry that reputation. i went on a trip with
www.flyingkiwi.com frmo a recommendation from a friend and it was outstanding. Met loads of cool people and found it quite flexible too, they even sorted out my transport to/from airport. only word of wanring is their trips are for people who like the outdoors, everyday i was kayaking, biking or doing something i thought it was a superb way to see nz and becuase they arent as well known as the others works out much cheaper.
otherwise its buy a car and do it yourself!