I've found that the O2 sim works well. You can buy it from us in Australia for $10 so you have your number to print on business cards and divert your phones to and within five minutes of clearing customs you can have it loaded with credit and active. The £2/day Euro roaming is a big improve...

abroadband data SIM cards can now be delivered outside Europe if you buy them from travelgear.com.au. The sim card costs $29 (with free delivery in Australia and New Zealand).