The iPhone Dev-Team, Chronic-Dev Team, and others try their jailbreaks on many iDevices before releasing them to make sure nothing bad will happen to the end user. ZiPhone was based off of stolen code and was used in an unsafe way. That's why some people had lasting issues with it. The only jailbreak stuff I'd suggest to stay away from are things made by iH8snow and GeoHot. Both of which make things that are sometimes buggy and unstable.
true but those dont do permeant damage.