I have been experimenting with belt drive for a number of years and I have a belt drive bike. It's homebrewed - uses a Thun & Pirelli set up.
The belt is nice and quiet. It certainly improves the sensation of cycling because it is quiet. I do not notice any difference in efficiency between it and a chain.
Belt drive has a lot of pluses going for it. Combined with a hub gear it is the obvious way forward.
Bikes will need some redesign before belt drive can become popular. Chainstays will have to be redesigned so there is absolutely no lateral flex, and also so that they separate at the dropout to allow the belt to be fitted.