Both mine and wife's are already paid for. At the rate they go up its one hell of a better return than leaving the cash in a bank or even the best investment I've come across. There was a bit on TV the other day on how much the charges have risen and a suggestion there may need to be some regulation.
Another argument for paying up front is that the council can't grab it if you end up in a home so it won't come out of the bit they leave over for the kids. The alternative scheme for hanging onto some cash makes the mattress very lumpy.