Am I mistaken in thinking that their efficiency is poorer/cost to run greater than horizontal radiators? I am replacing all my downstairs rads so don't want to make a costly mistake in the name of saving space.
We replaced all our crappy horizontal rads with vertical in our Victorian terrace and they work a treat. B&Q have a calculator to tell you if the particular radiator will heat the room effectively - definitely give that a go first before you buy.
I expect Pigletjohn will be able to tell you if there's science involved, but I can't see why a vertical set of pipes would work any worse than a horizontal set. We have verticals throughout and the house is very warm when they're on.