I have had an Iphone pretty much since they came out, bought from the states, unlocked to all networks and what is called 'Jailbroken' so you can install a massive library of unofficial software.
It is true that apple frown on such activity, and do indeed throw regular updates onto itunes to counter the unlocking procedure, but there is a work round within days to allow the update and keep the phone unlocked.
The phone itself is nothing short of fantastic , initially unable to send mms, record video,etc etc but all doable now with 3rd party software.
In short if you like /are able/happy to take the risk to re-flash your phone every now and again to keep up with the updates - then go for it, if not -go the official O2 route.