As an Aussie, who was like Koala very anti them I am now for them.
There is a big thread about them on the Aussie branch and I think it was Koala who was asking about them.
If you load them with the currency of the country you are travelling, you are locked into the exchange rate of that day. Can be good may be bad. There are no other charges for using them other than the initial purchase cost and the charge for putting more money onto it. Sorry there may be one, a charge for using an ATM for withdrawing cash, certainly not charged the international conversion charge (or whatever they call it) for each purchase. That may or may not be already in the exchange rate you got on purchase but if the exchange rate is fluctuating in your favour then it isn't a bother.
In Italy this year, I was using Aussie Dollars on it, only extra charge again was the ?$3.00 for withdrawing cash.
The only problem I have had with it, I accidentally used it as security for my hotel stay. Which at the time was something like £60 so didn't think it was going to be a bother. Unfortunately the Hotel stuffed up big time and managed to put a hold on over £300 of my funds, which left me without enough money to shop with. That could have also been a problem with an ordinary credit card, depending on funds.
The main advantage I see over an ordinary credit card, is being able to purchase your travel money when the exchange rates are good,even if you are getting a poor rate from the bank. For example, we have purchased Yen for my husband's upcoming trip, we purchased it at 93cents, it went higher so we have lost out there, haven't checked recently but think it has dropped below the rate we purchased - so then it was in our benefit.
Also, by loading in the currency you will be using it may be easier to keep track of your spending. ie load £1000 you can keep track of your receipts easier without having to do conversions.