I am jailbroken and loving it. It gives you a lot more freedom in customizing your iPhone. I have a minimal lock screen with a sleek looking clock, 5 app wide home screen and 5 app dock, and SB settings which allows me to turn off bluetooth, wifi, etc easily from the home screen. And it allows you use Facetime over 3G instead of just wifi. I also figured out how to make my own live animated wallpapers but they freeze up and just animation sometimes so I'm back to a normal wallpaper. Well, sort of normal. Its a pan wallpaper like an Android phone, the wallpaper scrolls with each page.
Check out
Macrumors.com, they have lots of info and tips.
Oh, and if you don't have unlimited 3G, there is
a way to get on an unlimited plan if you can use more than 2GB a month like me. Using Netflix over 3G eats up data really quickly, Pandora just eats it up over time.