i don't use them but i know that the charges are high. you pay a fee when getting the voucher and another fee to get any unused money returned to you. i was given the info a while back through top-up club.
Advantages:
1. handy
2. Can't spend more than you have.
3. Avoids high CC surcharges at Ryanair.com
4. Can track spending online.
5. No stamp duty.
Disadvantages
1. €350 limit. No use if buying a laptop from Dell.com
2. €5 charge per voucher, not too bad if buying high value vouchers.
3. €1.50 charge to redeem unspent credit.
4. Only valid for a max of 60 days, which is fine for most users.
5. No way to transfer funds from a used voucher to another so that you don't end up with unspendable small amounts that are simply lost to the bank. This is my major problem with them.
I found the 3V idea a great buzz when it first came out and even got one but the charge of €5 per voucher per top up is a bit excessive and specially as it seems €3.50 actually goes to the bank and the retailer receives €1.50.