I've bought my last 3 iPhones off contract although the previous 2 (3G & 3GS) weere still locked to O2 (UK).
Bought my 32GB 4 this morning and although I'll still be using it on O2 when in the UK it now gives me the flexibility to use it abroad on a local network rather than pay roaming costs.
Advantage of "off contract", I just sell the last model in 12 months time and pick up the latest, no upgrade hassles!
i am in the US and on Tmobile. if i can find one unlocked for a reasonable price i just might pick one up to avoid jailbreaking......not that jailbreaking is a problem but it does take a while to get the latest apple OS upgrade if you want to keep your unlock
I would say it depends how long you intend to stick with the Carrier? If you're going to be with that Carrier for the next two years which means you paid the full price for the phone and you're still paying the same amount for your voice and data plan for the same time you would have anyway why not get something out of the deal and let them absorb some of the cost of the phone. If you can afford it and you think you might jump ship to another carrier in time then just buy it unlocked. Also consider the unlocked version if you travel alot.