I haven't tried rooting yet but have always jailbroken my iPhone. Mainly for a couple tweaks that made the iPhone more functional.
SBSettings for toggles.
LockInfo for a full featured/customization notification center
3G Unrestrictor for faster internet browsing
FakeClockUp for faster transitions between screens
As well as some customizing of the layout and icons.
I'm still getting used to Android but I'm sure I'll eventually start rooting as well.