Three years after this post was published, I published an update here.
The most recent version of an air-powered car, first introduced by Guy Nègre in 1991, was featured on Shark Tank recently. Robert Herjavec is willing to invest $5 million into Zero Pollution Motors provided that they negotiate a North American license from Guy's company, Motor Development International. Currently, ZPM only has a license for Hawaii. Autoweek covered the story here.
Don't expect to see these cars on the road any time soon. Guy has been flogging this idea for over 20 years now with only prototypes to show for it. In 2007, he announced that Tata Motors had licensed the technology and would manufacture cars in India but a visit to Tata Motors' Web site shows no mention of air-powered cars. To my knowledge, Tata has not sold a single air-powered car.
Most people see the appeal of the concept. Where is the problem? It's in compressing the air for the car. Compressing air produces heat, which is typically lost, reducing efficiency. On the flip side, in the car, expanding the air absorbs heat which means the air needs to be heated, reducing efficiency. One way around this is to capture the heat and store it in a battery, using that energy to heat the air when it is used in the car. The end result is that an air-powered car is not sufficiently energy efficient to be practical.
I could see using air power in situations where energy efficiency is not a factor but convenience or access to compressed air is. For example, in an electrical power plant, the waste heat from the boilers could be used to compress the air, reducing the overall energy cost.
Part of the Shark Tank pitch was the concept of using "turnkey micro-production factories" to build the cars locally. With the car already being impractical, saddling it with an unrealistic manufacturing model will surely doom it. Just as Tesla has one manufacturing plant, only one manufacturing plant is needed to serve North America.
One final point: Neither Ethan Tucker or Pat Boone, the presenters on Shark Tank, appear to have any experience building or selling cars. Ethan, at least, is listed on the management page of ZPM.
I wish Mr. Nègre the best, but the physics are against him.