One more point for T-Mobile and roaming in Europe: they offer a
Euro Travel Booster where you get £7.50 credit for £5, valid for 20 days.
"Charge: £5. Allowance: £7.50 (up to 19 minutes of calls, receive up to 39 minutes of calls or send up to 30 texts)"
This might be handy since sometimes you may actually need to make or receive a call. I've contacted them asking whether this credit applies to GPRS as well. That would make their £1.50/MB for Europe even lower.