I absolutely do.
I was blown away that I couldn't readily change various things as I have always been able to do with previous cellphones on the same carrier (Rogers). The inability to lose the icons on the homescreen was maddening and the selection of wallpapers and ringtones was a bit on the lame side. Certain things I didn't ask about before buying because they have always been standard features (ie: applying own Ringtones) and I only found out that these features were actually disabled after trying to find controls for them in the Settings.
I have a hobby of designing a few wallpapers on the side and being able to actually see them, and not
just on the lockscreen, was a must (still can't understand why Apple did that). Now I have a blank homescreen and custom pages for each section - and I can see my wallpapers

.
Another sticky point with my
3G was my volume level, which was absurd. Now I can hear my iPhone ring from across the street with a nifty bit of code that was written to change the default volume values to increase it (actually, almost doubled it).
Lastly, though not vital, was that I can change the themes, icons, status bars, remove dock, battery, etc. which cannot be done otherwise.
With the age of being able to personalize a phone this way or that, Jailbreaking is the only way to pull this off with the iPhone.
Party on with the Dev Team!
