T-Mobile Euro Internet Booster - cheap data roaming in Europe

[f300]

This looks very promising for low usage.
I see them stating:

Pay per day customers
Make sure you top up before you go, and the cost will come off your balance.

What is "Pay per day customers"? Is it some kind of prepaid? If yes it would be appreciated if somebody could include some short info about how to get and how to recharge these cards for non-uk residents (if possible at all). This looks _very_ promising; it's better than any MVNO offering AFAIK and it's T-Mobile so it should work better than mostly anything else.