They make their money from the currency exchange rates when your sterling loaded card dispenses local currency or is used to pay for goods in the local currency. so do compare these rates before you commit with other card providers.
I would opt for a sterling card from Escape if you are planning on making a lot of ATM withdrawals and purchases as they have no fees and use fairly good exchange rates.
If you can keep the number of ATM withdrawals down and take out larger amounts then opt for Caxton who offer great rates and no ATM charges.
Fair FX is a good card overall as their exchange rates are generally one of the best in the market along with ICE so these can offset their purchase and ATM fees.
Click here for our recommendations http://www.moneysupermarket.com/prepaidcards/
Hope this helps.
Travel expert, www.travelsupermarket.com