I jailbroke but wasn't happy with the speed or the glitches it introduced (more crashes, even the slightest tap set off the icons wobbling to move them).
FUD.
Jailbreaking by itself has no effect on the iPhone. Certain apps that you install from Cydia CAN cause lag and other problems. Just be careful what you install.
It's like saying USING TERMINAL IN OSX CAN CAUSE KERNEL PANICS... well then don't use the Terminal in OSX because you can really screw things up.
Some people shouldn't be JBing their phone because they are idiots and install every unstable POS crap hack on the planet in Cydia.
My favorite app is PdaNet. That saves me tons of cash by having free internet on my laptop. Also the Installous app saves me hundreds of dollars since I can evaluate every App before paying for it.