To be honest, even though I've lost a little bit of money in the recent liquidisation of the Sheffield City Flying School; I would still consider paying for block hours up-front in the future. Its wouldn't be a definate action, but its something that I would consider based on the flying school/club.
In the end, it is cheaper to fly when one pays in block hours due to the discounts applied, and like all gambling you have to accept that you might lose. The only real question to be answered is whether the discount is worth the risk involved, and whether you can afford to lose the amount that you 'invest'.