I was jailbroken exactly once - with the jailbreakme.com jailbreak (sorry I've forgotten the name). The thing is, just during the first few seconds of booting, there were a few flashing pixels and IT MADE ME CRAZY that my wonderful user experience was somehow marred. I want to know that my user experience is perfectly and lovingly tuned by Apple's caring craftsmen.
For me it comes down to user experience. I came from a Windows Mobile phone. I had every software that you can imaging and it modded to perfection. The thing is - the user experience was just crap.
When I use my iPhone or iPad, I want to know that it is perfect, and while yeah, it annoyed me at first that I couldn't do stuff that I could on my WinMo phone, the experience and smoothness was just amazing! Since then, with iOS 4.2 and apps like GoodReader and AirVideo, I really have nothing more to complain about.
(this is a reprint from an email I sent to another guy at another blog)