Only thing is that o2 charge more for upgrade phones than what it costs if you start a new contract with them
But this does vary based on how much you've spent in the past 12 months (calls, your montly cost, etc), I paid £109 for my Nokia N73, plus a reduction of my tariff from £19 to £14 a month, only cos I couldn't be bothered to shop around and I've been happy with o2's service