An absolutely brilliant bit which I came across on T-Mobile's website:
Can I use the internet on my phone or mobile broadband in Europe without buying a Booster?
We have boosters to buy so that you'll always know exactly how much you've spent and know that you won't get any unexpected bills. Using the internet on your phone or mobile broadband in Europe without a booster would previously have been charged at £1.50 per MB, so 50MB would have cost you £75. With a booster a 50MB allowance is only £10 so you're getting much better value with boosters. You can buy as many boosters as you want and the cost of it will come out of your balance or added to your monthly bill but you'll know exactly what you're paying.
To me it looks absolutely beautiful. Fine, maybe this is the way it's being done in England and English people are used to it — then please forgive a clumsy foreigner, but something tells me this has got very little to do with English customs and traditions :)
On the other hand, I cannot praise enough very fair international data tariffs — £10 for 50MB is nothing to worry about, really. Unfortunately, it only works as long as you stay in Europe — but it is still way better I could've expect while being locked in with an O2.