I've owned a 2G, 3G, and a 3GS and have found jailbreaking to be a completely safe experience if you approach it like you approach everything else in life: with a little planning, reading, and thinking BEFORE you act.
I do have a few friends who eat up the Apple propaganda about jailbreaking being illegal and immoral and although I've tried to gently explain it to them that neither of those arguments are valid, they still think it's something that just isn't for them. At the same time, they're always impressed by the exponentially increased usability of my phone compared to theirs whenever they use mine.
I'm not saying everyone who doesn't jailbreak chooses not to do so for these reasons, but I have found a good amount of people are simply afraid of that which they don't understand.
Oh well, to each his or her own!