I've been to NZ many times, so already have a very good idea of what to expect, so I personally would always recommend visiting first. Over the years I have seen many people make the move without visiting who have eventually come to regret it and moved back to the UK. However, I also do know several people who didn't visit first and are very happily settled in NZ now. I guess really it comes down to what type of people you are and how easily you can adapt.
Words and pictures though don't give you a good enough idea of what it's really like.