It is really easy tbh.
Lost none of the functionality, and in some cases have added stuff that improves on the original software.
Originally, my brothers GF got one for her birthday (bought off eBay from the US) and it was unlocked already, but then it got bricked due to her upgrading the FW via iTunes.
Being the geek of the family, I said I'd have a go at "modding" it and spent 19 hours non-stop learning how to do it and managed to unlock it once again , but made it more stable.
Nowadays, there are some neat packages that take all the heartache out of unlocking an iPhone.
If you are interested to learn, then send me a PM, because I don't want to upset any mods or admins by posting something potentially illegal.