I have just come back from Germany (Lower saxony) and whilst I had coverage, both in rural and city areas I couldnt get the phone to work, it had continual E! no data etc. This was despite switching off and on, airplane mode off and on etc. I had to use my wife's phone. This was annoying as I have international 30 country coverage JUST for travel. I got coverage in the USA earlier this year but the goons who sorted my contract out at O2 put me on android and not apple (there is a difference as far as the network operators are concerned), so the phone worked but I was charged extra!!
They have refunded the extra charges I incurred from the USA but wont refund the vodaphone extra charge my wife will be charged. SO! Beware and make sure O2 have the correct phone if you're on the international bolt on and take the numbers to call to get issues sorted if, as in my case, your phone wont work on anything other then E, SO you can get help.
