if you want to live in a city, Auckland or Wellington are the best options. Auckland ranked 5th in the
Mercer worldwide quality of living survey which looks at thing such as weather, crime, infrastructure and general lifestyle. Wellington ranked 12th.
If you want to live somewhere that is chilled out and has nice weather, then Nelson is very popular, but it's a pretty small place.
Crime is generally low in New Zealand and it's a fairly safe place, but of course it has its problems. Gun crime here is very low relative to the US, it's just not part of the culture, police don't generally carry guns even.
Biggest difference I think most people from the US will notice is geography. New Zealand is very isolated, it's a long way from the rest of the world really, if you want to visit friends back in the US, it's a 12 hour flight just to the US west coast. Of course for some people the isolation is exactly why they like NZ.
any other questions hit me up on icq anytime. 739765
edit:
NZ property <- prices in auckland are expensive, you will still get an ok house for 500k though.