3 years ago, we bought a prepaid Mastercard through the Post Office. We used most of the money on the first year's holiday, and used the rest whilst away in France this year.
Not everywhere accepts the card, even though it's a mastercard. The petrol pumps we tried to use it in didn't accept it, but supermarket check outs did. We could also get money out of cash machines without any drama.
The only "restriction" was that we had to empty the card completely before the expiry date, which we managed to do last week in France.
Overall, I would regard such cards as a useful option, but I would not want to rely solely on one for a whole holiday.