The compressed air designation is very misleading. This is a hydraulic hybrid, using a hydraulic pump/motor the same way a normal hybrid uses a battery - for acceleration and storing braking energy.
Hydraulic hybrid vehicle (HHV) technology has been slowly maturing and is very promising. It is already in use for some advanced heavy trucks. Garbage trucks in particular, with their stop and go usage profile benefit from their efficiencies.
UPS is trying some out. Manufacturers like Navistar and Eaton are on board
In the U.S the EPA has been at the forefront of the research. See their page about it: Hydraulic Hybrid Research
In 2011 the EPA announced a partnership with Chrysler to produce an HHV minivan that would give you a 60% improvement in city driving fuel economy.
Hybrid Batteries are expensive and can't handle the braking energy a truck generates. Hydraulic technology is cheap, well understood, and gives you more bang for your buck.