If you want to use your iphone to tether to your laptop for an internet connection - or if you want to transfer files between two iPhones - then this is a great option.
I suggest you do it - google "BlackRain" - it will take you from here.
Keep in mind it voids your warranty - however, if you do a "Restore" on your phone, no one will know.
Many other things to do with a jailbroken iPhone - tethering and transfer via bluetooth are the most significant.