I think in Europe, they have actual SIMs, not these dongles, and you can get more favorable rates, like if you recharge 10 Euro each month, you get a couple of hundred megabytes or something like that.
So how do you re-up for another month, do they bill you or you just pay online by a credit card? The latter would give you the most flexibility. Or say it's the 15th of the month and you anticipate using 3G for a few days so you purchase for a month then. Next month, you don't need it and in fact, you might not buy data again for months. Can they maintain your account?
Better than these USB dongles would just be a SIM that you would drop into a MiFi type of device.