Depends largely on which provider you are with.
Some of them charge more per unit than both a normal quarterly bill and DD. Which is why PP is a last resort in regards to debt and not paying bills on time.
Also yes arknor, they will clear it if you call, but allot of people dont even notice and phone up complaining the meter is eating the money they put onto the card. Its mainly down to the customers own stupidity and not phoning straight away though.
I can only comment on my meters and British Gas made a massive song and dance about the price per unit was the same, I ended up having the meters taken out anyway but the unit price was the same.
I prefer PP in a rental as the flat we rented in London (moved out in June) and it took 2 months of fannying about to get the utility companies to sort out our final bills (and consequently our refunds as we'd vastly over-paid).
Also if I was a landlord I'd insist on PP meters as then massive bills cannot be run up.