Car sharing on the street - trust the Germans to do it more efficiently!

Car2go, a smart new way to organise car sharing - with cars parked on the street in ordinary parking spaces rather than in specially designated bays. Great if it works - but I wonder if it is inherently limited to little places like Ulm, where this one is. Would it work in a big city like London? Would you need too many cars if users could just leave them wherever they liked? I hope to follow up and see how this turns out.