Also, as far as jailbreaking, there are some cool things you can do that for me are essential:
1) access your phone's full file system from a phone file manager or your computer.
2) fast tweaks by just sliding the top bar, gives you a slide-down of every tweak on the phone (iOS5 stock still lacks this)
3) more notification center tweaks and widgets
4) Video Wallpapers
5) Gravity Board (youtube it, it's pretty awesome)
6) UI tweaks like adding an HTC animated clock to the home screen, a calendar widget, facebook widget, rss feed widget (on the springboard)
7) rotate the springboard to landscape, and 5 icons on the bottom instead of four
console game emulators playable by wii remote or PS3 remote
and there's much more theming and tweaking available.