well basically it lets you have 3rd party apps, MOST are free, and any you pay for are on paypal.
I can customise EVERY sound.
Save youtube ringtones.
Hide apps you cant remove (itunes,contacts,maps,youtube)
I have a video camera! (and the videos aint that bad to be honest)
I have bluetooth and can actually send/recieve things.
Video Ringtones.
theres 1000's of apps on there, its really easy to do, and if you dont like it, just do a restore on your iphone!
actually jailbreaking the phone takes about 20mins max!